Problem:
The previous fix in #34314 relies on copying the tree in `tree_root` to
ensure the `TSNode`'s tree cannot be mutated. But that causes the
problem where two calls to `tree_root` return nodes from different
copies of a tree, which do not compare as equal. This has broken at
least one plugin.
Solution:
Make all `TSTree`s on the Lua side always immutable, avoiding the need
to copy the tree in `tree_root`, and make the only mutation point,
`tree_edit`, copy the tree instead.
168bf0024e