Make root selectable + many other changes

[trueadm]

This PR does quite a few things to fundamentally improve how we handle selection:

• Root nodes can now be selected
• Decorator nodes can now be isolated from selection
• Most elements can not be correctly removed
• Selection can escape tables into the root
• HorizontalRule is now a decorator rather than a core base node and works better

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[zurfyx]

LGTM! We should probably let the iOS folks know about these changes since they've already mimicked a good part of this selection logic

[zurfyx]

Why do we need the Bootstrap plugin for now without params? Sounds like we should make them mandatory?

[trueadm]

Why do we need the Bootstrap plugin for now without params? Sounds like we should make them mandatory?

Where?

[zurfyx]

Why do we need the Bootstrap plugin for now without params? Sounds like we should make them mandatory?

Where?

export default function LexicalBootstrapPlugin({
  initialPayloadFn,
  clearEditorFn,
}: {
  clearEditorFn?: (LexicalEditor) => void,
  initialPayloadFn?: (LexicalEditor) => void,
}): React$Node {

[trueadm]

I didn't change them as part of this PR did I?