Trigger turbo:before-morph-added and turbo:before-morph-remove events

[ajoabraham]

It would be great if we can hook into node add and node remove events to apply custom animations or other processing. Currently we can only hook into before-morph-element or before-morph-attribute. These will not give us access to newly added nodes for custom processing.

Current default Idiomorph callback for beforeAdded:

turbo/src/core/morphing.js

Lines 61 to 63 in 5eca98f

	beforeNodeAdded = (node) => {
	return !(node.id && node.hasAttribute("data-turbo-permanent") && document.getElementById(node.id))
	}

[yujiteshima]

I've implemented this feature with 4 events instead of 2, following Turbo's existing event naming conventions:

Event	Cancelable	Description
turbo:before-morph-element-added	Yes	Fired before a new element is added
turbo:morph-element-added	No	Fired after a new element has been added
turbo:before-morph-element-removed	Yes	Fired before an element is removed
turbo:morph-element-removed	No	Fired after an element has been removed

This matches the existing pattern of turbo:before-morph-element / turbo:morph-element for morphing, and provides both cancelable "before" events and non-cancelable "after" events for add/remove operations.

I've opened PR #1482 implementing this feature with 4 events following Turbo's existing naming conventions.

Note about potential conflict with PR #1438:
My PR modifies src/core/morphing.js, which may conflict with #1438 (Add refresh method to Turbo Drive). I've reviewed both changes and the overlap appears to be minimal - likely just a few lines in the callback class. If #1438 is merged first, I'm happy to resolve any conflicts on my branch.

Demo: https://github.com/yujiteshima/turbo-morph-events-demo

PR: #1482

[ajoabraham]

Awesome! Thanks for doing this.