Property access for string index signatures

[sandersn]

Fixes #12596

This PR allows property access (dotted syntax) for arbitrary identifiers on types that have an explicitly declared string index signature. Previously, only element access (bracket syntax) was allowed. For example:

interface Flags { [name: string]: boolean }
let flags: Flags;
// new
flags.thisIsFine = true;
flags.imOkayWithTheEventsThatAreUnfoldingCurrently = true;
// still works
flags["That's okay, things are going to be okay."] = true;

Improvements:

1. Javascript says that property and element access are basically the same thing. Typescript does too now.
2. Property access now behaves the way Javascript programmers expect. Less friction when learning Typescript and less code that needs to be rewritten to use element access.

Drawbacks:

Types with string index signatures allow any property name. This means:

1. You won't get errors if you use a property that doesn't exist.
2. The compiler won't report errors for typos.

interface Map<T> { [s: string]: T }
let forgot: Map<number>;
// oops, forgot isn't an array!
for (let i = 0; i < forgot.length; i++) {
  console.log(forgot[i]);
}

let cantType: Map<string>;
cantType.something = 'soemthing';
if (cantType.soemthing) {
  console.log(cantType.soemthign);
  launchMissiles(cantType.soehign);
}

[mhegazy]

@sandersn can you refresh and merge?

[sandersn]

Done