Declarative extensions schema

[aborgna-q]

Declarative extensions are defined in YAML format, as shown in the spec.
#833 and #839 added support for these, but the specific format is not well defined as there is no schema for it in the spec.

Some issues:

• The spec example uses types "Q" and "USize" in the signature, but these are not real types. Should we have hardcoded aliases for the prelude types? Where are they defined?
• Name resolution. Currently we just match the name of the types directly on all extensions in scope, without namespacing.
  What's the name schema for them? Should there a way to bring all of an extension's definitions into the local scope? (I.e. to let us do qubit instead of prelude.qubit).
• There is no info on how the lowering definition works.