feat(treesitter): start moving get_parser to return nil #30313

**Problem:** `vim.treesitter.get_parser` will throw an error if no parser
can be found.

- This means the caller is responsible for wrapping it in a `pcall`,
  which is easy to forget
- It also makes it slightly harder to potentially memoize `get_parser`
  in the future
- It's a bit unintuitive since many other `get_*` style functions
  conventionally return `nil` if no object is found (e.g. `get_node`,
  `get_lang`, `query.get`, etc.)

**Solution:** Return `nil` if no parser can be found or created

- This requires a function signature change, and some new assertions in
  places where the parser will always (or should always) be found.
- This commit starts by making this change internally, since it is
  breaking. Eventually it will be rolled out to the public API.

test/functional/treesitter/parser_spec.lua

@@ -135,9 +135,7 @@ void ui_refresh(void)
     insert(test_text)
 
     eq(
-      '.../treesitter.lua:0: There is no parser available for buffer 1 and one'
-        .. ' could not be created because lang could not be determined. Either'
-        .. ' pass lang or set the buffer filetype',
+      '.../treesitter.lua:0: Parser not found.',
       pcall_err(exec_lua, 'vim.treesitter.get_parser(0)')
     )