Handle string escaping for SimpleString expression in FormattedString

[jimmylai]

f"{a or ""}" this is invalid syntax but LibCST handles it and renders the invalid code.

We can either:

• automatically escape the string quotes in the inner expression.
• raise exception on code generation.

[DragonMinded]

We should raise an exception. We do not do any mangling of constructor arguments on any nodes elsewhere and shouldn't attempt to do so here.