resolveType config parameter should be optional on StrongInterfaceTypeConfig type

[karoun]

As explained by @leebyron in this issue comment, there exists an alternative to the resolveType interface method in the form of isTypeOf methods on the objects implementing the interface (see also #3).

The resolveType parameter should be made optional to support this default behavior.

graphql-strong/src/interface.ts

Lines 36 to 45 in c85a0c7

	/**
	* The configuration object to be used when creating interface types. It
	* requires `resolveType` and `fields`.
	*/
	export type StrongInterfaceTypeConfig<TValue, TFieldMap extends StrongInterfaceFieldMap> = {
	readonly name: string,
	readonly description?: string | undefined,
	readonly resolveType: (value: TValue) => StrongObjectType<TValue, never>,
	readonly fields: StrongInterfaceFieldMapConfig<TFieldMap>,
	};