Add accessors for treesit-font-lock-settings

Add accessors for treesit-font-lock-settings

Since each SETTING in treesit-font-lock-settings is considered an opaque
object, provide accessor functions for each field.

* lisp/treesit.el:
(treesit-font-lock-settings): Update docstring.
(treesit-font-lock-setting-query):
(treesit-font-lock-setting-enable):
(treesit-font-lock-setting-feature):
(treesit-font-lock-setting-override): New functions.
(treesit--font-lock-setting-feature): Remove function.
(treesit--font-lock-setting-enable): Rename to
treesit--font-lock-setting-clone-enable to avoid confusion with
treesit-font-lock-setting-enable.
(treesit-add-font-lock-rules): Use renamed function.
(treesit-font-lock-fontify-region): Add a comment.
* doc/lispref/modes.texi (Parser-based Font Lock): Update manual.

(cherry picked from commit f9fa75c1ef31ee78b43509f15b50cf0603757181)