String interpolation #147

Some other programming languages (Groovy, JavaScript, Kotlin, ...) allow for string interpolation.
Check if we can/want to allow this as well.

String interpolation: $"xxx{variable}" is an interpolated string and can contain variables. Expressions are not (yet) supported and will be evaluated later. It is possible to escape the { and }. Escaping } is not necessary but permitted: $"This is literally a { and }"
It is not permitted to use an interpolated string as member name (x.$"something" is not allowed) since member names must be constants.

Acceptance criteria:

• Add String Interpolation feature
• Decide: Should this work for ..?
  • Single-Quoted Strings
  • Double-Quoted Strings
  • WYSIWYG-Strings (single or double quoted)
• Add test that checks the feature
  • Green path
  • Test(s) that test escaping the {} characters (or whichever chars are used to refer to variables.
  • reference a non-existing variable
  • invoke more complex logic inside the interpolation
  • have an interpolation expression that does not return a string (should that attempt an implicit/explicit cast to string or fail?)