Extract context/feature settings for use by main library

[Chris Fuller]

Some of the more invasive changes we would like to make really need some kind of feature flagging capability, which the core library currently lacks. The idea here is to move the FeatureFlagProvider and surrounding portions of the wiki markup Context up to the main library and wire it in as much as possible.

Given that there are many calls like copy(), validate(), and toMap() that are implemented on individual nodes and may be called in the wild, it may be quite some time before use of the library propagates through to all of the library functionality that might want to use it. The initial plan is to surface it on Doc and most of the AdfParser implementations for now and wait until it is actually needed for something to change the rest of it.