support extended css selectors in hx-partial

[MichaelWest22]

Description

Found we can easily support extended selectors in hx-partial targeting. This allows you to target elements relative to the triggering source element using all the htmx enhanced selectors like closest, previous, next etc.

Also fixed a bug in the same code where if you only have hx-partial's and they don't find valid targets it would not do the expected behaviour of blocking the empty main swap. we just need to make sure it always emits a partial task so the auto empty detection code works.

Corresponding issue:

Testing

added a basic test

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

[scriptogre]

I think it'd be more sane if <hx-partial> and hx-swap-oob behaved the same.

Can we combine this with #3926 and close one of the PRs?

[1cg]

Thank you Latent!