fix htmx.ajax defaulting to swap body when target not found

[MichaelWest22]

Description

The htmx.ajax helper function can cause issues when the target is not set correctly or the target selector happens to be missing in the DOM as it passes in null target override to the ajax request which defaults to targeting the body. Changing it to detect invalid targets and pass in a DUMMY_ELT which triggers the htmx:targetError solves this. Also had to handle situations where a valid source is supplied as its valid to fall back to source targeting an explicit target is not defined but if we still want it to fail if target was defined.

Corresponding issue:
#2869

Testing

Wrote some new tests to cover most invalid targeting situations and also tested calling htmx.ajax manually to confirm it rejects invalid targets but passes though valid ones fine.

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

[MichaelWest22]

rewrote the logic into an expanded if statement so its easier to understand and debug and added inline comment