Do not execute hx-trigger="load" on re-initialization of an existing node

[basvk]

Description

Sometimes, requests that should be only triggered once at load (with hx-trigger="load") are executed twice. This happens for example when calling htmx.process on an existing node (or a parent node) after changing an attribute on the element with hx-trigger="load".
This PR adds a firstInitCompleted flag to an element's internalData and adds a check for that flag when handling the load triggerEvent.

Corresponding issue:
#2973

Testing

Added a test scenario that changes hx-swap="afterbegin" to hx-swap="beforeend" on an element with hx-trigger="load", followed by a call to htmx.process and checks whether the innerHTML of the element stayed the same.
This test fails on the dev branch, and passes with this fix.

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

[Telroshan]

Just an idea out loud, not a change request, what would you think of "reversing" the check to simplify the code, i.e. only process the load trigger if the nodeData.something property is falsy, and only set that property to true after initialization ?
This would remove the need for that first true assignment in initNode as well as the undefined comparison, as a falsy check would cover it (so only assign it to true after initialization, and check that it's falsy in addTriggerHandler)

[basvk]

Thanks, good idea! I've changed firstInit into firstInitCompleted and removed that unnecessary check in initNode.