feat: use short comments

[Rich-Harris]

Closes #10609. So far, this just replaces <!--x--> with <!x>. I seem to recall there being some issue with JSDOM (or something) not recognising these as comments, but it appears to no longer be an issue.

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[trueadm]

I wasn't sure if this PR was a draft by accident, but it looks good. Other than the failing test.

[Rich-Harris]

Draft because I have a whole bunch of follow-up work planned, but I think it might be better to tackle things in stages, so I'll merge this once green

[michael]

Speaking of comments... I have no good idea for a syntax, but it'd be neat to be able to just comment out props temporarily. E.g.:

<Foo
 /*bar="bar"*/
 baz="baz"
/>