Fix Slow  Markdown Front matter stripping regexp (#19894)
Fix Slow  Markdown Front matter stripping regexp (#19894)

**Bug**
For long/not-terminated frontmatter in a markdown file, we can currently hang the process while trying match it using a regular expression

**Fix**
Use a more efficent regexp to do this.
Microsoft/vscode,KattMingMing/vscode,radshit/vscode,joaomoreno/vscode,rishii7/vscode,mjbvz/vscode,hashhar/vscode,rishii7/vscode,cleidigh/vscode,KattMingMing/vscode,the-ress/vscode,hungys/vscode,mjbvz/vscode,matthewshirley/vscode,radshit/vscode,Krzysztof-Cieslak/vscode,Krzysztof-Cieslak/vscode,eamodio/vscode,eamodio/vscode,hungys/vscode,Microsoft/vscode,rishii7/vscode,hoovercj/vscode,0xmohit/vscode,gagangupt16/vscode,radshit/vscode,hungys/vscode,the-ress/vscode,landonepps/vscode,veeramarni/vscode,radshit/vscode,Krzysztof-Cieslak/vscode,Microsoft/vscode,microsoft/vscode,matthewshirley/vscode,rishii7/vscode,joaomoreno/vscode,Microsoft/vscode,hashhar/vscode,eamodio/vscode,microsoft/vscode,hungys/vscode,hungys/vscode,mjbvz/vscode,landonepps/vscode,eamodio/vscode,Microsoft/vscode,eamodio/vscode,cleidigh/vscode,microsoft/vscode,the-ress/vscode,radshit/vscode,mjbvz/vscode,DustinCampbell/vscode,gagangupt16/vscode,radshit/vscode,rishii7/vscode,hungys/vscode,rishii7/vscode,joaomoreno/vscode,veeramarni/vscode,microsoft/vscode,KattMingMing/vscode,eamodio/vscode,microsoft/vscode,landonepps/vscode,matthewshirley/vscode,veeramarni/vscode,mjbvz/vscode,stringham/vscode,hungys/vscode,mjbvz/vscode,stringham/vscode,hashhar/vscode,cleidigh/vscode,hungys/vscode,the-ress/vscode,microlv/vscode,Zalastax/vscode,hoovercj/vscode,DustinCampbell/vscode,KattMingMing/vscode,DustinCampbell/vscode,DustinCampbell/vscode,Microsoft/vscode,Zalastax/vscode,Krzysztof-Cieslak/vscode,Zalastax/vscode,microsoft/vscode,stringham/vscode,Zalastax/vscode,the-ress/vscode,matthewshirley/vscode,hoovercj/vscode,DustinCampbell/vscode,0xmohit/vscode,Krzysztof-Cieslak/vscode,hungys/vscode,joaomoreno/vscode,eamodio/vscode,hoovercj/vscode,mjbvz/vscode,mjbvz/vscode,Microsoft/vscode,Zalastax/vscode,microsoft/vscode,veeramarni/vscode,landonepps/vscode,mjbvz/vscode,gagangupt16/vscode,stringham/vscode,Zalastax/vscode,hungys/vscode,eamodio/vscode,matthewshirley/vscode,Microsoft/vscode,stringham/vscode,DustinCampbell/vscode,microlv/vscode,veeramarni/vscode,DustinCampbell/vscode,microsoft/vscode,Krzysztof-Cieslak/vscode,microsoft/vscode,mjbvz/vscode,eamodio/vscode,rishii7/vscode,rishii7/vscode,0xmohit/vscode,KattMingMing/vscode,cra0zy/VSCode,the-ress/vscode,DustinCampbell/vscode,matthewshirley/vscode,radshit/vscode,0xmohit/vscode,Krzysztof-Cieslak/vscode,KattMingMing/vscode,cleidigh/vscode,stringham/vscode,landonepps/vscode,KattMingMing/vscode,matthewshirley/vscode,joaomoreno/vscode,joaomoreno/vscode,stringham/vscode,hashhar/vscode,the-ress/vscode,stringham/vscode,0xmohit/vscode,microsoft/vscode,hoovercj/vscode,eamodio/vscode,joaomoreno/vscode,Zalastax/vscode,joaomoreno/vscode,cleidigh/vscode,microlv/vscode,microlv/vscode,cleidigh/vscode,microsoft/vscode,DustinCampbell/vscode,joaomoreno/vscode,DustinCampbell/vscode,0xmohit/vscode,DustinCampbell/vscode,Microsoft/vscode,the-ress/vscode,veeramarni/vscode,Zalastax/vscode,veeramarni/vscode,joaomoreno/vscode,veeramarni/vscode,joaomoreno/vscode,landonepps/vscode,0xmohit/vscode,veeramarni/vscode,stringham/vscode,eamodio/vscode,rishii7/vscode,rishii7/vscode,KattMingMing/vscode,mjbvz/vscode,hoovercj/vscode,landonepps/vscode,Krzysztof-Cieslak/vscode,Zalastax/vscode,microsoft/vscode,veeramarni/vscode,cleidigh/vscode,joaomoreno/vscode,matthewshirley/vscode,microlv/vscode,hashhar/vscode,radshit/vscode,Krzysztof-Cieslak/vscode,microlv/vscode,Zalastax/vscode,0xmohit/vscode,joaomoreno/vscode,microsoft/vscode,microlv/vscode,veeramarni/vscode,0xmohit/vscode,veeramarni/vscode,cleidigh/vscode,gagangupt16/vscode,0xmohit/vscode,KattMingMing/vscode,hoovercj/vscode,matthewshirley/vscode,microlv/vscode,gagangupt16/vscode,KattMingMing/vscode,joaomoreno/vscode,gagangupt16/vscode,mjbvz/vscode,joaomoreno/vscode,gagangupt16/vscode,landonepps/vscode,rishii7/vscode,veeramarni/vscode,joaomoreno/vscode,hoovercj/vscode,gagangupt16/vscode,Microsoft/vscode,radshit/vscode,hoovercj/vscode,Zalastax/vscode,0xmohit/vscode,radshit/vscode,landonepps/vscode,landonepps/vscode,gagangupt16/vscode,matthewshirley/vscode,rishii7/vscode,rishii7/vscode,gagangupt16/vscode,Krzysztof-Cieslak/vscode,0xmohit/vscode,landonepps/vscode,veeramarni/vscode,rishii7/vscode,microsoft/vscode,radshit/vscode,hashhar/vscode,Zalastax/vscode,stringham/vscode,eamodio/vscode,gagangupt16/vscode,hashhar/vscode,microlv/vscode,stringham/vscode,hashhar/vscode,hoovercj/vscode,the-ress/vscode,hashhar/vscode,KattMingMing/vscode,gagangupt16/vscode,hoovercj/vscode,matthewshirley/vscode,KattMingMing/vscode,stringham/vscode,Krzysztof-Cieslak/vscode,Krzysztof-Cieslak/vscode,hungys/vscode,mjbvz/vscode,hungys/vscode,mjbvz/vscode,matthewshirley/vscode,hungys/vscode,matthewshirley/vscode,hashhar/vscode,0xmohit/vscode,0xmohit/vscode,hashhar/vscode,gagangupt16/vscode,cleidigh/vscode,matthewshirley/vscode,0xmohit/vscode,gagangupt16/vscode,the-ress/vscode,microsoft/vscode,microlv/vscode,KattMingMing/vscode,Krzysztof-Cieslak/vscode,Microsoft/vscode,hungys/vscode,hungys/vscode,Zalastax/vscode,joaomoreno/vscode,mjbvz/vs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