Rewrite global object initialization to delegate to ES

Points of note:

- ECMAScript now has the appropriate hooks (as of tc39/ecma262#226) for a separate global this value and global object, allowing us to fix a long-standing willful violation.

- Worker initialization was refactored extensively, to avoid creating WorkerGlobalScope objects before the corresponding realm was created. (The existing setup had the problem where it was creating a WorkerGlobalScope, derived from Object, in a thread that didn't even exist yet.)

- The "JavaScript global environment" concept was hackily removed. As discussed in #380 and #382, the only uses of "JavaScript global environment" are for a couple of canvas-related concepts which need to be removed anyway. And the definition we were using did not line up very well with ES's. We temporarily replace them with vague phrasing "lives in a window" and "lives in a worker"; this is OK since those sections will be deleted soon. This fixes #380 and fixes #382.

This is roughly steps 3, 4, and 6 of https://www.w3.org/Bugs/Public/show_bug.cgi?id=25981. It contains all the changes necessary to integrate with ES's new initialization mechanisms for a given host-defined realm, without yet any of the changes for script execution. This also fixes https://www.w3.org/Bugs/Public/show_bug.cgi?id=27419.