Add ability to trigger an event on another element using HX-Trigger response header

[bencroker]

Description

This PR adds the ability to trigger an event on another element using the HX-Trigger response header (also HX-Trigger-After-Swap and HX-Trigger-After-Settle) by allowing a target argument.

HX-Trigger: {"myEvent": {"target": "#myElement"}}

This addition was discussed with @1cg before submitting this PR.

Testing

Added a test to ensure the event is triggered on the target element.

Note that the @ts-ignore is required to prevent Typescript complaining that the target property does not exist on never. If there’s a better way, please let me know!

I tried using detail['target'] but that’s no better!!

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

[gijsleussink]

Beautiful improvement. 👍 In combination with takeClass it gives the opportunity to "activate" an element after a network call. In my case a list item.

What if the target element cannot be found? I have a list of 60k items (i.e. keywords in a dictionary) that are not all loaded on first request.
Shouldn't absence of the target element be handled? I think I now get a strange error.

[bencroker]

I think I now get a strange error.

What error?