using standard polyfill wrapping pattern to access global scope in multiple runtimes

[caridy]

Accessing window object directly is not recommended because it bound this polyfill to a browser. Other polyfills are using a more elegant solution to access the global scope.

With this PR, we can guarantee:

• using this (which is window in the browser, and global in nodejs) as the global scope to augment it with Intl member.
• preserving OldIntl on the global scope (even though it is not used in this script at the moment).
• a new Intl will be created everytime that this script gets evaluated.

[andyearnshaw]

Yeah, this was always the plan but I never got around to doing it. I'll merge for now and consider other module wrappers when I have more free time.

[caridy]

@ericf didn't get the chance to update the PR to use global instead of globals, @andyearnshaw already merged it. I used globals to match the CLDR polyfill, which apparently defines that as globals to avoid confusions, but anyway, it is just a local variable.

[ericf]

@andyearnshaw thanks for merging this!