Python support

[ethan-leba]

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Due to Python's whitespace-based parsing, there are oftentimes nodes with identical boundaries. For example, {foo;} in Java would simply be foo in Python, but the structure still implicitly exists. The node in the hierarchy affects what editing operations are valid, so that's something to keenly be aware of working in Python.

TODO:

• [x] Complete tree-edit-syntax-snippets for all nodes (d151e7f)
• [x] Add keybindings in tree-edit-nodes for all nodes (d151e7f)
• [x] Disallow empty blocks (ab07ec5)
• [x] Fix tree-edit--parse-fragment parsing expressions as expression statements (deaac61)