Fix incompatibility with tree-sitter-javascript >= 0.20.2

Fix incompatibility with tree-sitter-javascript >= 0.20.2

Starting from version 0.20.2 the grammar's primary expression
"function" has been renamed to "function_expression".  A new
function checks if the new primary expression is available,
and if so, it returns the correct rules.
* lisp/progmodes/js.el
(js--treesit-font-lock-compatibility-definition-feature): New
function.
(js--treesit-font-lock-settings): Use it.  (Bug#68879)