Add an intermediate Placeable production for Expressions within Patterns

[stasm]

We currently don't expose the information about the exact position of the { … } placeable in the source. Consider:

foo = foo { bar }
      ^_^          TextElement span
            ^_^    MessageReference span
      ^---------^  Pattern span

The suggested solution is to add an intermediate Placeable node for Expressions which are elements of Patterns:

foo = foo { bar }
      ^_^          TextElement span
            ^_^    MessageReference span
          ^-----^  Pleaceable span
      ^---------^  Pattern span