Fix event not being available in hx-vals/hx-vars when hx-trigger has delay

[scrhartley]

Description

When using hx-vals (and hx-vars) you can reference event as shown in the docs for hx-vals.
However when a delay modifier is used in hx-trigger this stops being true, since it's actually a reference to window.event, rather than an argument being passed. This is surprising and also its documentation says:

Deprecated: This feature is no longer recommended. Though some browsers might still support it, it may have already been removed from the relevant web standards, may be in the process of being dropped, or may only be kept for compatibility purposes. Avoid using it, and update existing code if possible; see the compatibility table at the bottom of this page to guide your decision. Be aware that this feature may cease to work at any time.

Corresponding issue : #1189

Testing

New tests added for hx-vals (I didn't copy them for hx-vars, but I fixed it for parity)

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