Cancel vanilla submits from <button[form]/>.

[geoffrey-eisenbarth]

Description

HTMX wasn't cancelling vanilla form submission from buttons with form attributes (e.g., outside of <form>).

Corresponding issue: #2919 can probably be closed with this

Testing

Added testing in test/core/internals.js. After a bit of back and forth on this, Carson agreed that expanding the existing test was appropriate.

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