Add 3 more beauty contestants

[stasm]

After some more thinking and offline discussions, I'd like to extend the beauty contest (#485) with the following 3 approaches:

• 0a is a hybrid mode which allows for simple patterns to be unquoted (...) Renamed to 2a.

• 1b is a variant of the text-first 1, but with the syntax directly inspired by existing templating languages.

  {% match {$foo} {$bar} %}
  {% when foo bar %} Hello {{ $foo }} you have a {{ $var }}
  {% when * * %} {{ $foo }} hello you have a {{ $var }}
  

• 2a is a hybrid mode which allows for simple patterns to be unquoted (Hello!), and switches to the "complex" mode when the message is wrapped in {{ }}. The complex mode then uses the current syntax verbatim.

  {{ match {$foo} {$bar}
     when foo bar {Hello {$foo} you have a {$var}}
     when * * {{$foo} hello you have a {$var}}
  }}
  

• 3b is a variant of keyword-based 3, but it uses #[...] for all control-flow-related syntax.

  #[match {$foo} {$bar}]
  #[foo bar] Hello {$foo} you have a {$var}
  #[  *   *] {$foo} hello you have a {$var}