Allow EntityManager.create to infer EntityInstance type

[bengreenier]

Hi folks - I wanted to be able to omit the Entity type parameter on EntityManager.create, since I believe it can be inferred without issue. This PR attempts to allow just that.

Previously:

const user = new User();
user.id = '1';
user.name = 'Test User';
user.status = 'active';

const ret = await manager.create(user);
// ^? const ret: unknown

Now:

const user = new User();
user.id = '1';
user.name = 'Test User';
user.status = 'active';

const ret = await manager.create(user);
// ^? const ret: User

Please let me know what you think, and thanks for your hard work!

[bengreenier]

since there is now a relationship between the type parameter Entity and the first parameter, we actually can't keep this test exactly as is.

The options I see are:

• drop the User type parameter altogether
• drop the EntityInstance cast
• use EntityInstance as the type parameter and keep the EntityInstance cast on the input
• drop all the type parameters and rely on inference (which will be inferred as the POJO type)

I choose to drop the cast, as that felt the cleanest to me, in expressing the failure intent (you can't pass POJO objects in when an Entity instance is expected).