Support `[arg()]` pattern matching

[nk9]

https://just.systems/man/en/attributes.html?highlight=arg%20pattern#attributes

[arg('flag', pattern='--help|--version')]
info flag:
  just {{flag}}

[arg('bar', long)]
@foo bar:
  echo bar={{bar}}

[arg('bar', long, brassy)]
@foo bar:
  echo bar={{brassy}}