Add a new Statement/Expression.raw(...)

[matanlurey]

... to allow opt-outs for more advanced scenarios we won't/can't cover with builders.

Example use:

new ExpressionBuilder.raw('5 + 90 >> 2 | 3')

new StatementBuilder.raw(r'''
  switch (value) {
    case: ...
  }
''')

We also need to be able to support auto-scoping - we could use mustache:

new StatementBuilder.raw(r'''
  switch ({{value}}) {
    case 1:
      break;
    case 2:
      break;
  }
''')

Would ~roughly translate to:

switch ($scoped.value) {
  ...
}