feat(cli): add stdout implicit arg type

CLIs can now use this for implicit validation for options that take a
file path or - to signal "print to stdout", like so:

  (defcli! (doom command) ((outfile ("--out" (file stdout))))
    (if (equal outfile "-")
        (print! "output")
      (with-temp-file outfile
        (insert "output"))))

If OUTFILE is not an existing file path or a -, you'll see an this
helpful error:

  Error: -o/--file received invalid value "FOO"

  Validation errors:
  - Must be a dash to signal stdout.
  - File does not exist.

  See 'doom h[elp] make codeowners' or 'doom make codeowners {-?,--help}' for documentation.

lisp/doom-cli-lib.el

@@ -62,6 +62,10 @@ See argument types in `doom-cli-argument-types', and `defcli!' for usage.")
             :read doom-path
             :error "File does not exist"
             :zshcomp "_files")
+    (stdout :test ,(lambda (str) (equal str "-"))
+            :read identity
+            :error "Must be a dash to signal stdout"
+            :zshcomp "(-)")
     (path   :read expand-file-name :zshcomp "_files")
     (form   :read read)
     (regexp :test ,(lambda (str) (always (string-match-p str ""))))