Allow strings inside string interpolation

[Goju-Ryu]

I have noticed that interpolations with strings inside are treated as unterminated interpolations. I have found that I have had to introduce a lot of extra bindings to avoid this when writing functions working with strings. I had the following usecase: "{str_replace(value_str_value_part, ".", "") }".
This currently isn't possible so I had to either introduce a named variable in an already cluttered function or give the static strings variables outside the function to use the names instead of the value directly. Some other common usecases I have had for this feature when working in other languages has been something similar to "{if true then "True" else "False"}".

[sharkdp]

If we can make this work without introducing ambiguities, let's do it.