Add support for hx-trigger on boosted elements

[xhaggi]

Description

Since htmx 4, boosted <a> and <form> elements ignore hx-trigger. They always fire on click (links) or submit (forms), and trigger modifiers like delay, throttle, once, from, or event filters have no effect.

Corresponding issue: #4126

Testing

I added two tests boosted link should be triggered by custom event and boosted form should be triggered by custom event to test/tests/attributes/hx-boost.js.
It fails without the fix and passes with it.

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

[xhaggi]

The test failure is unrelated to my changes, or is it?

The browser was unable to create and start a test page after 30000ms. You can increase this timeout with the browserStartTimeout option.