[SuperEditor] - Make all DocumentNodes immutable (Resolves #2166)

[matthew-carroll]

[SuperEditor] - Make all DocumentNodes immutable (Resolves #2166)

For undo/redo and edit transactions to be trustable, the document and its nodes need to be immutable from outside the Editor. This PR makes that conversion.

Replacing nodes

By making nodes immutable, each node can no longer be directly updated for things like changing text, indent, etc. Instead, every time a change is made to a node, the previous node must be replaced by the new node in the document. For that reason, a lot of the changes in this PR is focused on switching from direct node manipulation to document node replacement.

This also means that tests need to re-query nodes throughout the test. We should avoid storing nodes as intermediate values in tests, whenever we can, so that we don't accidentally try to look for changes on the same immutable node.

New copy methods

Moving from direct editing of a node to node replacement, we need the ability to copyWith() changes. Unfortunately, there's not a clean solution to this because Dart doesn't support method overloading. As a result, to deal with subclasses, we have to declare things like copyTextNodeWith(), copyParagraphWith(), and copyTaskWith(). Moreover, each subclass needs to adjust the superclass copy method so that subclass instances don't lose some of their data when the parent class' copy method is called.

[matthew-carroll]

FYI @miguelcmedeiros @brian-superlist @jmatth @Jethro87 - This PR will likely break some things for you. The fixes should be pretty simple and straight forward, but some changes will likely be necessary for you.

Changing document nodes to immutable brings us one step closer to capturing snapshots for undo/redo.