Font-lock shorthands in elisp-mode for quick visual recognition (bug#50959)

Font-lock shorthands in elisp-mode for quick visual recognition (bug#50959)

Only the shorthanded prefix is font-locked.  This allows the remainder
of the font-lock logic to subsist (e.g. for macro-defining symbols).

* lisp/shorthands.el (cl-lib): Require it when compiling.
(elisp-shorthand-font-lock-face): New face.
(shorthands--mismatch-from-end): New helper.
(shorthands-font-lock-shorthands): New helper.

* test/lisp/progmodes/elisp-mode-resources/simple-shorthand-test.el:
Add some dummy test code.