Access `global` as a type

Since https://github.com/Microsoft/TypeScript/wiki/What%27s-new-in-TypeScript#augmenting-globalmodule-scope-from-modules it's been possible to augment the global from a module. However, in TypeScript definitions (such as dom.d.ts or node.d.ts) we're left to create our own "globals". This results in two issue:

1. inconsistency with what is actually the global namespace
2. requiring the declaration of global types multiple times in case it's used as a "real global" or from window. (browsers) or global. (node)

Instead of this, it'd be really nice to access TypeScript's global as a type. For instance, in dom.d.ts we could do:

declare var window: global

Or in node.d.ts:

declare var global: global

I couldn't see any previous issues, but it's partially related to things like https://github.com/Microsoft/TypeScript/issues/12902 - the difference though is that those issues seems to be tracking adding variables to the TypeScript global scope whereas this would be using the global scope as a type to define these variables themselves.

Start by reviewing the proposal alongside dom.d.ts and node.d.ts, then read the related issue #12902 to distinguish this request from global-scope additions. Done means TypeScript provides a usable global namespace type so declarations such as window and global can reference it without duplicating global types.