Calling `operator+` over `string` and `char` errors: Missing compiler required member System.String.Concat

[isc30]

For some reason, when using operator+ from string type like the following:

return "" + str[0];

the compiler complains:

missing compiler required member System.String.Concat

but when using this it all works fine:

return "" + "b";

I suspect this is due to DSharp not supporting implicit conversion between char and string for this case

[isc30]

JS does not support char type and it returns a string when you do str[0], so we could define extern operator+ in dsharp for both string and char and it will work out of the box.

Maybe since we are already here, define Concat member function too that calls the static one.