feat(hx-trigger): add support for rootMargin on intersect

[believer]

Description

Add support for rootMargin in intersect triggers. This allows us to extend the size of the intersect area. I've selected to separate multiple values using underscores, similar to how Tailwind solves it.

Corresponding issue: #1349

Testing

I've tested it locally using one of my own projects. I didn't find any tests for the intersect modifiers, so I couldn't find anything to base new tests on.

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

[1cg]

i like everything about this except the underscores. What if we consumed until EOF or the next token followed by a colon? (this may be a horrible idea idk)

[believer]

I wasn't too excited about the underscores either, but I didn't have a better idea at the time. Two other options could be:

• /,|(root)|(threshold)/ to match comma or one of the other two intersect options
• /[,rt]/ for comma and the starting character of the options

I think the first option is easier to understand though

[believer]

@1cg any thoughts on the updated parsing proposal above?