Add interpolation syntax (f-syntax) for strings

[kodek16]

Add a new string literal form that looks like f"...". Inside this literal, $ character has a special meaning: it must be followed by an expression that is evaluated, converted to string, and inserted in the string constructed from the literal at evaluation time.

Two interpolation patterns should be supported: $name evaluates the variable named name, while ${x} evaluates an arbitrary expression x.