Add rx extension mechanism

Add rx extension mechanism

Add a built-in set of extension macros: `rx-define', `rx-let' and
`rx-let-eval'.

* lisp/emacs-lisp/rx.el (rx-constituents, rx-to-string): Doc updates.
(rx--builtin-symbols, rx--builtin-names, rx--local-definitions)
(rx--lookup-def, rx--substitute, rx--expand-template)
(rx--make-binding, rx--make-named-binding, rx--extend-local-defs)
(rx-let-eval, rx-let, rx-define): New.
(rx--translate-symbol, rx--translate-form): Use extensions if any.
(rx): Use local definitions.
* test/lisp/emacs-lisp/rx-tests.el (rx-let, rx-define)
(rx-to-string-define, rx-let-define, rx-let-eval): New.
* etc/NEWS (Changes in Specialized Modes and Packages):
* doc/lispref/searching.texi (Rx Notation, Rx Functions, Extending Rx):
Add node about rx extensions.