Prepare to not localize private function descriptions

The overwhelming majority of localizable messages comes from
completions:

	$ ls share/completions/ | wc -l
	$ 1048

OTOH functions also contribute a small amount, mostly via their
descriptions (so usually just one per file).

	$ ls share/functions/ | wc -l
	$ 237

Most of these are private and almost never shown to the user, so it's
not worth bothering translators with them. So:

- Skip private (see the parent commit) and deprecated functions.
- Skip wrapper functions like grep (where the translation seems to
  be provided by apropos), and even the English description is not
  helpful.
  - Assume that most real systems have "seq", "realpath" etc.,
    so it's no use providing our own translations for our fallbacks.
- Mark fish's own functions as tier1, and some barely-used functiosn
  and completions as tier3, so we can order them that way in
  po/*.po. Most translators should only look at tier1 and tier2.
  In future we could disable localization for tier3.

See the explanation at the bottom of
tests/checks/message-localization-tier-is-declared.fish

Part of #11833

(cherry picked from commit d835c5252a)

share/functions/__fish_cmd__complete_args.fish

@@ -1,4 +1,5 @@
-function __fish_cmd__complete_args -d 'Function to generate args'
+# localization: tier3
+function __fish_cmd__complete_args
     set -l current_token (commandline -tc)
 
     switch $current_token