Further separates them from `IBeatSnapProvider`'s `SnapTime`, and groups them together more, to prevent confusion between the two interfaces.
Also changes the xmldoc of the reference time to that of `IBeatSnapProvider` for consistency.
Seems `EditorBeatmap` already implements a different kind of `SnapTime` from `IBeatSnapProvider`, so method names here aren't great.
This is very similar to what https://github.com/ppy/osu/pull/12558 is doing, so may need to do some duplicate resolution later, especially surrounding `ClosestBeatSnapDivisor`.
Worth noting that this change makes 1/7, 1/5, etc unsupported for now, as we now rely on `BindableBeatDivisor.VALID_DIVISORS`.
