LibWeb: Parse HTML fragments directly into DocumentFragment

[shannonbooth]

Refactor the HTML fragment parser to insert the root's children straight
into a DocumentFragment instead of a connected temporary document. On
top of being a correctness fix, this should also improve performance of
fragment parsing.

Making this change has other fallout due to the timing of document
adoption changing. This results in related other changes needing to be
made, including:

• The parser having "allow declarative shadow roots" as a parser flag
  so that it does not read the flag from the wrong document. This
  prevents some regressions which would happen otherwise, but also
  aligns our behaviour in general where in some cases the wrong setting
  was previously being chosen.
• Custom element registry is now taken from the target node instead
  of the context node. This also prevents regressions as part of the
  fallout of these changes, but also fixes the selection of the custom
  element registry for ShadowRoots (which was previously being taken
  from the root document).
• Use a null custom element registry for template.innerHTML. Similar to
  both cases above, this prevents regressions from the above changes
  due to the change in timing of document adoption, but fixing this
  also aligns behaviour closer what other engines implement.

See: whatwg/html#12624 for background.

[github-actions]

Your pull request has conflicts that need to be resolved before it can be reviewed and merged. Make sure to rebase your branch on top of the latest master.

[shannonbooth]

fixed da conflicts