crates/triggers: Support argument expansion with :all/:each

[joebonrichie]

Currently, trigger handlers execute once per matched path, this can be slow for trigger handlerss which support processing multiple args.

Introduce argument expansion which allows triggers to batch arguments

• $(var) / $(var:each) - Execute handler per match
• $(var:all) - Execute handler against batched matches

e.g. instead of
systemctl preset foo.service
systemctl preset bar.service
systemctl preset baz.service

Do:
systemctl preset foo.service bar.service baz.service

[joebonrichie]

Alternative to #701, we get practically the same performance uplift with the systemctl preset handler changed to use :all