Revise tree-sitter facility for multi-language buffers

Revise tree-sitter facility for multi-language buffers

* doc/lispref/parsing.texi (Multiple Languages): Extend and update
manual.
* lisp/treesit.el (treesit-range-functions): Remove variable.
(treesit-range-settings): New variable.
(treesit-range-rules): New function.
(treesit--merge-ranges): New function.
(treesit-update-ranges): Use treesit-range-settings instead of
treesit-range-functions.
(treesit-font-lock-rules): Fix docstring.
(treesit-indent)
(treesit-indent-region): Only update ranges in a region.
* test/src/treesit-tests.el (treesit-range): New test.