allow definition to be introduced by keyword "define"

[Sven Verdoolaege]

It's not visually obvious that

theory T : V {
  {
    A("a").
  }
}

is different from

theory T : V {
  A("a").
}

The proposal would be to allow

theory T : V {
    define {
        A("a").
    }
}

Hendrik complained about that before.