Design of multi-schema support

[effigies]

In #94 (comment) / bids-standard/bids-specification#1981, @yarikoptic proposes a breaking change to the schema context. This would change both the context loader and the fields that the context needs to expose to expressions.

Other breaking changes could include:

• Addition of new structures in the schema that need to be looked up by name (objects.<newobjecttype>), as opposed to iterated over (rules.files.raw.<newdatatype>).
• ... (to be updated)

This issue is for working through possible mechanisms for supporting older and newer schemata in the same code base. This would allow us to avoid excessive code divergence between main, which is constrained to support the latest BIDS release, and dev, which is constrained to support the latest BIDS development state.