POC on a new comment on a PR or issue only #2

Focusing only on editing a PR comment, the easiest case.

• the textarea should autosize with the same quality as OEM
• the font should match OEM
  • this way preview and edit are more in sync
  • the Overtype defaults don't like variable-width font because of bold and italic
    • we can just not highlight bold or italic at all, that will work pretty well
    • links, bullets, headers, and quotes are the most valuable wins
    • someday can explore color to signify bold and italic
• triple-backtick syntax highlighting of major languages will just work for short lines
  • could be turned off for long lines

The point of this is to be a strict-improvement to the OEM experience in 95% of cases. Okay if 5% of cases are better off without it.