Add ModelTypeMap for GraphQL object type materialization

[FionaBronwen]

Summary

Introduces ModelTypeMap for converting TypeSpec Models to GraphQL types, integrates it with the registry, and wires up the schema emitter to register and materialize models.

Changes

ModelTypeMap (src/type-maps/model.ts):

• Extends TypeMap<Model, GraphQLObjectType | GraphQLInputObjectType>
• Creates GraphQLObjectType for output types, GraphQLInputObjectType for input types during model materialization
• Uses usageFlag in TSPContext to determine which type to create
• Placeholder mapPropertyType() for future type resolution

Registry (src/registry.ts):

• Added modelTypeMap alongside enumTypeMap
• addModel(model, usageFlag) - registers a model with usage context
• materializeModel(name) - lazily materializes to GraphQL type
• materializeSchemaConfig() now includes models in output

Schema emitter (src/schema-emitter.ts):

• model handler calls registry.addModel()
• exitModel handler calls registry.materializeModel()
• Models now appear in emitted GraphQL schema

Example output

type Book {
  name: String
  page_count: String
  published: String
  price: String
}

type Author {
  name: String
  books: String
}

Note: All fields map to String as placeholder - full model property type mapping is TODO.