Add anychar' as alias to anything' in rx (bug#37659)

* lisp/emacs-lisp/rx.el (rx--translate-symbol, rx--builtin-symbols, rx):
* test/lisp/emacs-lisp/rx-tests.el (rx-atoms):
* doc/lispref/searching.texi (Rx Constructs):
* etc/NEWS:
Add `anychar', an alias for `anything'.  Since `anychar' is more
descriptive (and slightly shorter), treat it as the preferred name.

test/lisp/emacs-lisp/rx-tests.el

@@ -184,8 +184,8 @@
                  "ab")))
 
 (ert-deftest rx-atoms ()
-  (should (equal (rx anything)
-                 ".\\|\n"))
+  (should (equal (rx anychar anything)
+                 "\\(?:.\\|\n\\)\\(?:.\\|\n\\)"))
   (should (equal (rx line-start not-newline nonl any line-end)
                  "^...$"))
   (should (equal (rx bol string-start string-end buffer-start buffer-end