Update parseHTML to use Document.parseHTMLUnsafe where supported

[lukewarlow]

Description

Updates parseHTML to use Document.parseHTMLUnsafe where supported

This allows HTMX to support declarative shadow dom

Corresponding issue: #2682

Testing

Please explain how you tested this change manually, and, if applicable, what new tests you added. If
you're making a change to just the website, you can omit this section.

Checklist

• I have read the contribution guidelines
• I have targeted this PR against the correct branch (master for website changes, dev for
  source changes)
• This is either a bugfix, a documentation update, or a new feature that has been explicitly
  approved via an issue
• I ran the test suite locally (npm run test) and verified that it succeeded

[lukewarlow]

Not sure how best to test this?

[lukewarlow]

You can compare specs at https://html.spec.whatwg.org/multipage/dynamic-markup-insertion.html#dom-domparser-parsefromstring and https://html.spec.whatwg.org/multipage/dynamic-markup-insertion.html#dom-parsehtmlunsafe

These should be identical behaviour wise, except perhaps for the URL setting of the resultant document but I don't believe that's an issue for HTMX's use case.

[lukewarlow]

cleanInnerHtmlForHistory it'll be interesting to see if this might need updating, currently it returns innerHTML which serialises in a way that won't include declarative shadow dom. It might be that we need to swap that for getHTML()

[alexpetros]

I support this!

[1cg]

@MichaelWest22 is going to put this behind a config flag to ensure no semantic changes for existing users. Thank you!