Add hx-partial tag handling to v2

[MichaelWest22]

Description

This adds a back port of the hx-partial tag into htmx 2. It is a clean simple addition that just adds the parsing and required and a new findAndSwapPartials funciton that runs after processing OOB's. The only other core change is to prevent the main swap if the response is only white space after the new hx-partials are detected as the only swap contents. This leaves OOB behaviour unchanged so there should be no breaking changes. Also adds some new events and support to extend and use hx-xxxx tags via a htmx:processTemplate event.

Corresponding issue:

Testing

Added new tests for hx-partial

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