Refactored History into its own plugin

[amyworrall]

Refactored History into its own plugin

[amyworrall]

Notes:

• History (i.e. undo/redo) is now inside its own plugin, which is in turn inside its own library
• The playground now supports undo/redo
• In order to fit the new buttons on the playground toolbar, I had to refactor things a bit, bringing the styling (i.e. bold, italic etc) buttons into a popup menu.
• LexicalView no longer provides an EditorHistory object. It was confusing before, because that is only one part of what is required to implement undo/redo, so its presence might have suggested to developers that they didn't need to do any extra work. With the new system, where undo/redo is provided by a plugin, developers genuinely don't need to do any extra work, apart from writing the UI for the buttons that expose undo/redo to the user!

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