feat(runtime): add slotchange event and assignedNodes / assignedElements methods for scoped: true slots

[johnjenkins]

What is the current behavior?

Stencil's polyfilled - 'Pretend slots' - don't act like their native counterparts:

1. There's no way to get the nodes that are slotted (assignedNodes() / assignedElements())
2. There's no way to know when the slotted nodes have changed (slotchange event)

At present, on a scoped: true component devs have to resort to clunky mutation observers (for text, comment and element nodes on a default slot), saving node references to internal state to track when they change.

This is frustrating as Stencil, internally, knows what is slotted.

Making this knowledge available to users will enable them to remove substantial LOC and mirroring the the native APIs makes swapping between native & proprietary behaviour much simpler.

GitHub Issue Number: #3782

What is the new behavior?

Fixes #3782

assignedNodes and assignedElements methods have been added to the mock <slot /> nodes (which in reality is an an empty text node (or a comment node in debug mode) or a <slot-fb /> if there is fallback nodes).

Documentation

Does this introduce a breaking change?

• Yes
• No

No - although, there is a change in behaviour .. Previously when doing something like <slot ref={s => this.slot} /> ... the ref would resolve to a slotted node. I think this behaviour is 1) unintuitive & 2) not documented.

Testing

New WDIO tests

Other information

[johnjenkins]

@christian-bromann I could put this behind a new extras config flag.. WDYT?

[christian-bromann]

As always, fantastic job 🚀