Forbid custom keywords that start with `$`

[gregsdennis]

The Core keywords are easily identifiable by the fact that they start with $.

The specification recommends against defining custom keywords which also start with $, but this recommendation seems to be repeatedly ignored by implementations ($data seems to be a common violator).

While the "$" prefix is not formally reserved for the Core vocabulary, it is RECOMMENDED that extension keywords (in vocabularies or otherwise) begin with a character other than "$" to avoid possible future collisions.

Since this recommendation has been in the spec for multiple iterations, maybe it's time to move this to a MUST NOT requirement.

[handrews]

We avoided doing that because plenty of implementations support weird custom $-prefixed keywords. However, if we remove support for non-vocabulary keywords, I can see forbidding other vocabulary keywords from using it. Those older cases were non-vocabulary extensions.

[gregsdennis]

We've decided that unknown (non-vocab) keywords will be unsupported moving forward (pending some additional options).