Add EnumTypeMap for GraphQL enum materialization

[FionaBronwen]

Summary

This PR introduces EnumTypeMap, a concrete implementation of the TypeMap abstraction for converting TypeSpec Enums to GraphQL EnumTypes, and refactors the registry to use it.

Changes

EnumTypeMap (src/type-maps/enum.ts):

• Extends TypeMap<Enum, GraphQLEnumType>
• Uses the mutated type's name as the cache key
• Materializes GraphQL enum types from TypeSpec enums

Registry refactoring (src/registry.ts):

• Replace inline TSPTypeContext and materialization logic with EnumTypeMap
• addEnum() and materializeEnum() now delegate to EnumTypeMap
• Add allRegisteredNames set for future cross-TypeMap duplicate detection