Explicitly export schema namespace

This makes it more explicit that we are exporting that namespace.
It will also fix an error when using `eslint-plugin-import` with `import-resolver-typescript`.
As it only considers explicitly exported names to be exported.

index.d.ts

@@ -1,4 +1,4 @@
-declare namespace schema {
+export namespace schema {
   export type StrategyFunction<T> = (value: any, parent: any, key: string) => T;
   export type SchemaFunction = (value: any, parent: any, key: string) => string;
   export type MergeFunction = (entityA: any, entityB: any) => any;