Global scope clarifications in response to TC53 feedback

[andreubotella]

Here's the relevant feedback:

• The standard assumes that Window exists in the global scope ("The global object on Window-like and worker environments should always be an instance"). However, it does not include Window in section 5 (Common API Index).

• The text on the Worker global scope omits the worker global postmessage. But, it also does not mention that workers are obliged to include some, but not all, of the other properties of the global scope. What about importScript? Further, the text implies Worker support is optional ("If a web-interoperable runtime supports workers") – is it? My guess is it is required but that a runtime can document that deviation if it is unsupported. The text is ambiguous.

[jasnell]

LGTM with caveats. I'm happy with this going forward as is but would prefer the workers link to be added at least.