fix: include submit button name/value when button owns hx-post

[MichaelWest22]

Description

fix: include submit button name/value when button owns hx-post (#4073)

In htmx 4, __collectFormData relied solely on evt.submitter to append a submit
button's name/value. This is null on a click event — only a native form submit
populates it — so buttons with hx-post directly on them never sent their own
name/value.

Fix: if submitter is not set and elt is itself a named submit button inside a
form, treat elt as the submitter.

submitter ??= (form && elt.type === 'submit') ? elt : null;

This restores the htmx 1.x/2.x behaviour where a submit button's name/value is
always included in the request body.

Corresponding issue:
#4073

Testing

Added tests for various cases

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