Add blank Github issue template
Add blank Github issue template

This creates a basic "Blank Issue" [Github issue template](https://help.github.com/en/github/building-a-strong-community/configuring-issue-templates-for-your-repository) along the lines of the one [used by wasmtime](https://github.com/bytecodealliance/wasmtime/issues/new/choose). My primary motivation for this initial change is to prompt an alternative of private reporting of security issues, but over time we could add more templates that will help structure general reports.
fastly/lucet,fastly/lucet,fastly/lucet,fastly/lucet,fastly/lucet