#2056 Updated the tracking of changed values to record the value against th…

[mikeedwards83]

Description

Updated the tracking of changed values to record the value against the source element instead of the target element.
This allows multiple source elements to listen to a single target for a change.

Corresponding issue:

#2056

Testing

Please explain how you tested this change manually, and, if applicable, what new tests you added. If
you're making a change to just the website, you can omit this section.

I tested this against the example in the issue. With the fix both triggers were fired, not just the one.

I also performed a couple of tests against other HTMX components I had written to check I hadn't broken anything else. This wasn't extensive testing.

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