feat!(treesitter): do not return changes from LanguageTree:parse()

Never return the changes an only notify them using the `on_changedtree`
callback.

It is not guaranteed for a plugin that it'll be the first one to call
`tree:parse()` and thus get the changes.

Closes #19915

runtime/doc/news.txt

@@ -232,7 +232,10 @@ The following deprecated functions or APIs were removed.
   changes need to be contributed there first.)
   See https://github.com/neovim/neovim/pull/20674.
 
-- 'hkmap', 'hkmapp' and 'aleph' options were removed. Use 'keymap' option instead.
+• 'hkmap', 'hkmapp' and 'aleph' options were removed. Use 'keymap' option instead.
+
+• |LanguageTree:parse()| no longer returns changed regions. Please use the
+  `on_changedtree` callbacks instead.
 
 ==============================================================================
 DEPRECATIONS                                                *news-deprecations*

runtime/doc/treesitter.txt

@@ -1099,7 +1099,6 @@ LanguageTree:parse({self})                              *LanguageTree:parse()*
 
     Return: ~
         TSTree[]
-        (table|nil) Change list
 
 LanguageTree:register_cbs({self}, {cbs})         *LanguageTree:register_cbs()*
     Registers callbacks for the |LanguageTree|.

runtime/lua/vim/treesitter/languagetree.lua

@@ -228,7 +228,6 @@ end
 --- determine if any child languages should be created.
 ---
 ---@return TSTree[]
----@return table|nil Change list
 function LanguageTree:parse()
   if self:is_valid() then
     self:_log('valid')
@@ -302,18 +301,12 @@ function LanguageTree:parse()
   })
 
   self:for_each_child(function(child)
-    local _, child_changes = child:parse()
+    child:parse()
-
-    -- Propagate any child changes so they are included in the
-    -- the change list for the callback.
-    if child_changes then
-      vim.list_extend(changes, child_changes)
-    end
   end)
 
   self._valid = true
 
-  return self._trees, changes
+  return self._trees
 end
 
 --- Invokes the callback for each |LanguageTree| and its children recursively