systemd-storage-fs: convert to VERB_COMMAND and add --introspect-cli

The custom help() is replaced by a COMMAND() description, and
--introspect-cli is added. The abstract is changed from the noun phrase
"Simple file system backed storage provider" to the imperative "Expose
regular files and directories as storage volumes.", following the man
page and consistent with the other commands.

test/units/TEST-74-AUX-UTILS.introspect-cli.sh

@@ -52,6 +52,7 @@ INTROSPECTABLE=(
     systemd-socket-proxyd
     systemd-stdio-bridge
     systemd-storage-block
+    systemd-storage-fs
     systemd-tty-ask-password-agent
     systemd-veritysetup
     timedatectl