[2.0] Improve extension registration logic

[Renerick]

Description

This PR improves htmx extension registration by allowing extensions to contribute to the selector used in findElementsToProcess. This change makes extension initialization much simpler since it can just inform htmx about what elements it want to process without the need to look for them in processed fragments. This change nicely avoids double processing issue since deduplication logic is implemeted by querySelectorAll

Corresponding issue: bigskysoftware/htmx-extensions#16

Testing

Since 2.0 repo has no extension, tests for this behavior are added to corresponding extension directory over at bigskysoftware/htmx-extensions

see bigskysoftware/htmx-extensions#18

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