Indexing strings

[HalidOdat]

ECMASCript feature
Just like arrays you can also index the characters in a string.

Example code
This code should now work and give the expected result:

let x = "hello";
x[0]

The expected output is 'h'. the actual output is undefined.

This happens because the internal slots are not set for string objects. #591

[RageKnify]

I believe this was implemented in #1291, seems to be working as far as I can tell, if something is misisng we can open the issue again.