Improve type safety of name global variable

[falsandtru]

name global variable is still assignable to string type. So name is still unsafe. name: string | undefined works better than current definition name: never.

TypeScript Version: master

Code

const n1: string = <never>name; // pass
const n2: string = <string | never>name; // pass
const n3: string = <string | undefined>name; // error, expected

Expected behavior:

declare const name: string | undefined;

Actual behavior:

declare const name: never;

[falsandtru]

related: #9850 @DanielRosenwasser

[falsandtru]

@mhegazy @DanielRosenwasser Can you please also review this?

[mhegazy]

@DanielRosenwasser any thoughts

[mhegazy]

The resons we made it never was that ppl were stepping on it by mistake..

class C {
    name: string;
    method() {
        doSomething(name); // not this.name
    }
}

```
by making it `never` at least you will get an error and you will go back and check the source of the declaration. 

`string | undefined` will put us back where we started for most users.

[mhegazy]

as noted in #15424 (comment), never was added for a specific reason. It is much more common for users to use the global name by mistake vs intentionally. this change was meant to alert users in these cases; unfortunately it breaks other cases, but we believe this gets the 90% case right. for other cases, window.name.