Add operator (+) should support strings and compile to concat function

**Is your feature request related to a problem? Please describe.**
To improve building strings and perhaps enable multi-line experience similar to one that is present in C#, we could use `+` sign to concatenate strings.

**Describe the solution you'd like**
Currently expression `var x = 'a' + 'b'` gives error, while it could be compiled to `concat` function.

Use the issue's expression example as the starting point and locate the compiler entry points for binary operators and the concat function. Confirm how string operands should be handled and add coverage showing that `var x = 'a' + 'b'` compiles as concatenation; done means the expression no longer errors and produces the expected concatenated value.

Written by the indexing model from the issue text.