New Flymake backend using the shellcheck program

New Flymake backend using the shellcheck program

See bug#57884.

* lisp/progmodes/sh-script.el: Require let-alist and subr-x when
compiling.
(sh--json-read): Helper function to deal with possible absence of
json-parse-buffer.
(sh-shellcheck-program, sh--shellcheck-process,
sh-shellcheck-flymake): Variables and function defining a Flymake
backend.
(sh-mode): Add it to 'flymake-diagnostic-functions'.