Make named window lookup more precise and correct BCG swap logic

In "the rules for choosing a navigable," the method to find an existing navigable by name is vague. This updates the definition to accurately reflect what the major implementations do. There are some differences between implementations, so there remains in the spec some optional/implementation-defined behavior, but it's much narrower. In particular, note that lookups are now explicitly scoped to browsing context groups. The previous language in the named lookup about "the user agent determines that the two browsing contexts are related enough" is now no longer a part of the lookup logic, but a consequence of the BCG swap decisions.

In "obtain a browsing context to use for a navigation response," the existing spec only mentions COOP enforcement as a reason to do a browsing context group swap. Some implementations perform a swap for additional security and performance reasons. This is now reflected in the spec.

For context, see:

* #313
* #4198 (comment)
* #5350

This closes #313, but we have opened the following issues to track the remaining implementation-defined interop gaps: #6356, #10842, #10848, #10849, #10850.

Co-authored-by: Domenic Denicola <[email protected]>