Allow tree-sitter indent offset to be a variable

Allow tree-sitter indent offset to be a variable

* doc/lispref/modes.texi (Parser-based Indentation): Update manual.
* lisp/progmodes/js.el (js--treesit-indent-rules): Change all
occurance of ,js-indent-level to js-indent-level.

* lisp/progmodes/ts-mode.el (ts-mode--indent-rules): Change all
occurance of ,ts-mode-indent-offset to ts-mode-indent-offset.
* lisp/treesit.el (treesit-simple-indent-rules): Change docstring.
(treesit-simple-indent): Allow offset to be a variable.