Avoid parsing some Eshell forms when performing completion

Avoid parsing some Eshell forms when performing completion

During completion, we want to evaluate most Eshell forms
(e.g. variable references), but skip others (e.g. globbing,
subcommands).  For globbing, we want to pass the literal glob to
Pcomplete so it can use the glob for selecting completion candidates.
For subcommands (including Lisp forms), we especially want to avoid
evaluation, since they can produce arbitary side effects!  (Bug#50470)

* lisp/eshell/esh-cmd.el (eshell-allow-commands): New variable...
(eshell-commands-forbidden): New error...
(eshell-named-command, eshell-lisp-command): ... use them.

* lisp/eshell/em-cmpl.el (eshell-complete--eval-argument-form):
Disallow command forms and handle errors ourselves.
(eshell-complete-parse-arguments): Don't parse glob characters.

* test/lisp/eshell/em-cmpl-tests.el
(em-cmpl-test/parse-arguments/unevaluated-subcommand)
(em-cmpl-test/parse-arguments/unevaluated-lisp-form)
(em-cmpl-test/parse-arguments/unevaluated-inner-subcommand)
(em-cmpl-test/file-completion/glob, em-cmpl-test/command-completion)
(em-cmpl-test/subcommand-completion): New tests.
(em-cmpl-test/parse-arguments/pipeline): Remove superfluous
let-binding.
(em-cmpl-test/file-completion/after-list): Use a list variable rather
than a subexpression; the latter is no longer evaluated during
completion.
(em-cmpl-test/lisp-function-completion): Check "$(func)" syntax.