AST editing

[TwitchBronBron]

Adds ability to edit AST during beforeFileTranspile, and then the changes are reverted after the afterFileTranspile event.

• Adds new AstEditor class that applies edits and tracks how to undo them.
• Adds editor property to the beforeFileTranspile and afterFileTranspile lifecycle events.
• Once the afterFileTranspile event has fired for all plugins, the editor.undoAll() method is called which will revert all of the AST changes made during the transpile flow.
• Update docs with example
• any files that were edited using editor will have the needsTranspiled property set to true automatically.