Make template strings substitution parse as Elm, rather than injecting Elixir

[wende]

Right now you can put elixir into your expression using

mytest : String
mytest =
  "something something dark force #{DateTime.utc_now}

Which is not exactly pure.
Rather than that we need to parse the #{} tags and compile their insides too
So that

mytest : String
mytest =
  "something something dark force #{String.join "," [1,2,3]}"

[wende]

Blocked by #352