fix(treesitter): ensure TSLuaTree is always immutable

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.

runtime/lua/vim/treesitter/_meta/tstree.lua

@@ -26,6 +26,7 @@ function TSTree:root() end
 ---@param end_col_old integer
 ---@param end_row_new integer
 ---@param end_col_new integer
+---@return TSTree
 ---@nodoc
 function TSTree:edit(start_byte, end_byte_old, end_byte_new, start_row, start_col, end_row_old, end_col_old, end_row_new, end_col_new) end
 

runtime/lua/vim/treesitter/languagetree.lua

@@ -1107,8 +1107,8 @@ function LanguageTree:_edit(
   end_row_new,
   end_col_new
 )
-  for _, tree in pairs(self._trees) do
-    tree:edit(
+  for i, tree in pairs(self._trees) do
+    self._trees[i] = tree:edit(
       start_byte,
       end_byte_old,
       end_byte_new,