Add handling of contextual entities in tree-sitter font-lock

Add handling of contextual entities in tree-sitter font-lock

* lisp/progmodes/python.el: Remove function.
(python--treesit-settings): Capture contextual node.
* lisp/treesit.el (treesit--set-nonsticky):
(treesit-font-lock-contextual-post-process): New functions.
(treesit-font-lock-fontify-region): Change local variable START and
END to NODE-START and NODE-END, handle special capture name
"contextual".
* doc/lispref/modes.texi (Parser-based Font Lock): Update manual.