sh-script.el: Improve consistency in variable-use fontification.

sh-script.el: Improve consistency in variable-use fontification.

sh-mode--treesit-settings: Ensure all variable use is treated the
same way.
Before this patch the following commands would all be treated differently:

    echo "${var}"
    echo "$var"
    echo ${var}
    echo $var

Now they are treated the same.
* lisp/progmodes/sh-script.el (sh-mode--treesit-settings): Tweak
rules.  (Bug#78167)

(cherry picked from commit 7265be09cebc9ee752b688b7a85fb1ac18b45ef5)