fix: do not read a $ref inside default, const, enum or examples as a reference

[nigrosimone]

addSchema walks every object in a schema and records each $ref it meets, so a $ref key inside a default, const, enum or examples value, where it is a property of the instance and not a reference, is recorded as one. getSchemaDependencies then throws Cannot resolve ref for it, and getDerefSchema walks into the data too. It came up in fast-json-stringify: a schema with default: { $ref: '...' } on one property and anyOf on another cannot be built.

The fix skips those four keywords in both walks.

The added test puts a $ref under each of the four and checks the refs, the dependencies and the deref schema: it fails on main and passes here.