Converted part of scan_html_tag to C# from Regex
Converted part of scan_html_tag to C# from Regex

This does not have any measurable impact to parsing spec.txt but it
should have performance benefit when parsing documents with lots of HTML
elements.

lostfictions/CommonMark.NET,dudb/CommonMark.NET,AMDL/CommonMark.NET,laedit/CommonMark.NET,AMDL/CommonMark.NET,laedit/CommonMark.NET,dudb/CommonMark.NET,Mosoc/CommonMark.NET,lostfictions/CommonMark.NET,Mosoc/CommonMark.NET,dut3062796s/CommonMark.NET,dut3062796s/CommonMark.NET