fix: trigger filters on events with properties that aren't identifiers

[nickhstr]

Description

The keys used for an hx-trigger filter include every "for...in" property of an event. When the event carries a property whose name isn't an identifier, the Function constructor throws a SyntaxError.

__executeJavaScript now passes only the keys a function parameter can take: identifiers that aren't reserved words. A filter couldn't reference any other key by bare name anyway.

Corresponding issue: #4123

Testing

Added a new test case for hx-trigger.

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