PR: Add partial edits support for CodeEditor with LSP integration

[hlouzada]

Description of Changes

• Wrote at least one-line docstrings (for any new functions)
• Added unit test(s) covering the changes (if testable)

• Included a screenshot or animation (if affecting the UI, see Licecap)

• Added a EditsStackMixin to track the editor changes and provide external undo/redo support for CodeEditor;
• Captured EditBlock changes are merged together until a timeout occurs which commits the current pending edits by creating a QUndoCommand and pushing to QUndoStack for undo/redo;
• Each individual text changes are represented by a TextDelta object, that allows normalization, splitting complex edits, and merging adjacent deltas.
• The EditBlock contains the text deltas, the cursors states before and after the edit and a timestamp.
• CursorState is used to store the main and extra QTextCursor for undo/redo cursors support and continuity checks;
• Updated CodeEditor class to incorporate EditsStackMixin, replacing direct undo/redo calls with undo_stack.undo() and undo_stack.redo().

Affirmation

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I affirm the Developer Certificate of Origin
with respect to all commits and content included in this PR,
and understand I am releasing the same under Spyder's MIT (Expat) license.

I certify the above statement is true and correct: @hlouzada