[Expose] Expose ModelType Enum (#584)

This PR exports the `ModelType` enumeration defined in `config.ts` so
that clients could use it to differentiate different types of models.

Author: Neet-Nestor

src/index.ts

@@ -4,6 +4,7 @@ export {
   ChatOptions,
   MLCEngineConfig,
   GenerationConfig,
+  ModelType,
   prebuiltAppConfig,
   modelVersion,
   modelLibURLPrefix,