Infer string literals types in const declarations

[Daniel Rosenwasser (DanielRosenwasser)]

This PR experimentally adds functionality so that string literals are given string literal types in const declarations. It is the first step towards exploring "mutability widening" as brought up in the design meeting.

[Mohamed Hegazy (mhegazy)]

this still does not answer cases like:

const blue = "blue";
const red= "red";

let color = blue;
if (Math.random()) {
    color = red; /// Error !
}

in #5740 we talked about handling let and const differently, and allowing for different widening rules.

[Daniel Rosenwasser (DanielRosenwasser)]

I don't think this is the right thing to do for now.