Infer string literals types in const declarations

[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.

[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.

[DanielRosenwasser]

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