Fontize $(...) slightly better in bash mode

* lisp/progmodes/sh-script.el (sh-font-lock-keywords-var): Fontize
$(...) slightly better (bug#42417).  Instead of just fontizing the
first word in the expression, fontize until the closing
parenthesis.  This doesn't work well if you have nested $(...)
expressions.

lisp/progmodes/sh-script.el

@@ -838,7 +838,7 @@ See `sh-feature'.")
          font-lock-variable-name-face))
 
     (rc sh-append es)
-    (bash sh-append sh ("\\$(\\(\\sw+\\)" (1 'sh-quoted-exec t) ))
+    (bash sh-append sh ("\\$(\\([^)\n]+\\)" (1 'sh-quoted-exec t) ))
     (sh sh-append shell
 	;; Variable names.
 	("\\$\\({#?\\)?\\([[:alpha:]_][[:alnum:]_]*\\|[-#?@!]\\)" 2