New tree-sitter toggle scheme

New tree-sitter toggle scheme

This version: central variable, everything controlled by
treesit-settings.  Major mode sets up tree-sitter/non-tree-sitter
in a conditional branch, based on the setting.

* lisp/treesit.el (treesit-settings): New option.
(treesit-defun-type-regexp): Change docstring.
(treesit-mode-supported)
(treesit-required-languages)
(treesit--local-variable-backup): Remove variables.
(treesit--backup-local-variable)
(treesit-mode)
(global-treesit-mode--turn-on)
(global-treesit-mode): Remove functions.
(treesit--setting-for-mode): New function.
(treesit-ready-p): New argument MODE, changed REPORT to QUIET, and
LANGUAGEs to LANGUAGE (now it can be a single symbol or a list of
them).
(treesit-major-mode-setup): New function.  Mostly comes from
treesit-mode.

* test/src/treesit-tests.el (treesit-misc): New test.

* lisp/progmodes/python.el (python-mode): Move some setup code into
the conditional branch at the end.

* lisp/progmodes/js.el (js-json-mode)
(js-mode): Move some setup code into the conditional branch at the
end.

* lisp/progmodes/ts-mode.el: Move tree-sitter setup into the
conditional branch.