In shell scripts, case branches traditionally end with ;;. Bash
additionally supports case branches ending with ;& and ;;&. Zsh similarly
supports case branches ending with ;& and ;|. Currently sh-script.el
supports case branches ending with ;;, ;&, and ;;&, but not with ;|. The
attached patch adds support for case branches ending with ;|.

I have tested the patch by defining all the modified functions
(sh-smie-sh-rules, sh-font-lock-paren) and constants (sh-smie-sh-grammar,
sh-smie-rc-grammar, sh-smie--sh-operators, sh-smie--sh-operators-re,
sh-smie--sh-operators-back-re) in my .emacs (in a (with-eval-after-load
'sh-script ...) statement).

Here is an example indented without the patch:

case $input in
    *a* ) echo A;;
    *b* ) echo B;&
    *c* ) echo C;;&
    *d* ) echo D;|
*e* ) echo E;;
esac

and with the (simulated) patch:

case $input in
    *a* ) echo A;;
    *b* ) echo B;&
    *c* ) echo C;;&
    *d* ) echo D;|
    *e* ) echo E;;
esac

The first change in the patch replaces an (eq (char-before) ?|)
<https://github.com/emacs-mirror/emacs/blob/77ca6aa56e3425c87861cab8abce52bee3697cf4/lisp/progmodes/sh-script.el#L1045>
with
(and (eq (char-before) ?|) (not (eq (char-before (1- (point))) ?\;))). It
is needed to avoid confusing ;| tokens with plain | tokens. I wonder
however whether there would be a cleaner way of expressing the same.

The second change replaces a (looking-at ";[;&]\\|\\_<in")
<https://github.com/emacs-mirror/emacs/blob/77ca6aa56e3425c87861cab8abce52bee3697cf4/lisp/progmodes/sh-script.el#L1056>
with (looking-at ";\\(?:;&?\\|[&|]\\)\\|\\_<in"). The original expression
is looking for ;; and ;& tokens but not for ;;& tokens, which looks like an
oversight to me. That's why I have changed it to look for ;;, ;&, ;| but
also ;;&.

The other changes simply add support for ;| where there was previously
support for ;;, ;&, and ;;&.

Philippe