browsingContext.create should be able to support WebDriver classic New Window command

[bwalderman]

To support future efforts to reformulate WebDriver Classic on top of WebDriver BiDi, it should be possible implement New Window in terms of browsingContext.create.

There are a couple small gaps in the current spec PR #133

• The classic command uses the implicit current browsing context to determine which OS window the implementation should try to create the new browsing context in. However, BiDi doesn't have a concept of a "current" or "default" browsing context (by design).
• The classic command result value has a type that indicates whether the new browsing context was created as a "tab" or a "window". This depends on whether the new context is created in the same OS window as the current browsing context or a different OS window.

We'll likely need to add a parameter to the browsingContext.create command that the WebDriver classic spec can use to explicitly set the desired target window. Exactly how this looks will depend on whether we decide to introduce OS-level windows as a concept in WebDriver BiDi.

If we do add a "Window" concept like this, the browsingContext.create command could accept an optional "window" parameter and use that as a hint of where to create the new browsing context. The reformulated WebDriver classic spec could maintain a handle to the current browsing context's parent "Window" to use for this purpose. Otherwise, if we don't add a "Window" concept, we can still support the needs of WebDriver classic by adding a "context" parameter.