Add doctype to webview html

[cbreeden]

Reference: #7626

Not having a doctype declaration can cause some CSS renderings to do weird things. This is seen for instance in KaTeX as described in #7626. This will allow KaTeX to render properly in .previewHTML invocations.

[mention-bot]

By analyzing the blame information on this pull request, we identified @jrieken to be a potential reviewer

[msftclas]

Hi @cbreeden, I'm your friendly neighborhood Microsoft Pull Request Bot (You can call me MSBOT). Thanks for your contribution!

This seems like a small (but important) contribution, so no Contribution License Agreement is required at this point. Real humans will now evaluate your PR.

TTYL, MSBOT;

[rebornix]

@cbreeden thanks for your help on this, it's a valid bug fix and we ran into this issue several times and felt really painfull! We've had a fix for this issue while implementing Markdown Extension Extraction, see https://github.com/Microsoft/vscode/blob/kieferrm/markdown/src/vs/workbench/parts/html/browser/webview.html#L109 . Sorry for the late reminder :)

I'm Okay to have this merged first, totally up to @jrieken . If we want to merge this PR, my only comment is use another contentDocument.write instead of string concat.

[cbreeden]

Oh nice. I was writing a markdown editor extension myself when I came across this. No need to merge this if @jrieken is on his way to merging the Markdown Extension. I suppose this means that we will be able to change out the markdown rendering engine and even perhaps have access to those buttons in on top of the documents. Exciting stuff :) I'll wait, thanks for the response. 👍

[jrieken]

Thanks. My plan is to merge the markdown branch this week.