types: allow calling SchemaType.cast() without parent and init parameters

[vkarpov15]

Fix #14748
Re: #9076

Summary

The doc and init parameters to cast() should be optional in TypeScript, we have plenty of tests that call SchemaType.prototype.cast() with only 1 arg. Plus #9076 recommends using Schema.Types.Subdocument.prototype.cast() as an alternative for instantiating a subdocument without an associated parent document, which is the root cause of the issue in #14748.

Examples

[hasezoey]

Looks good, some slight suggestions

[vkarpov15]

@hasezoey please take a look at the new implementation, I added a test based on your comments, and also some more logic to allow passing DocType down to the Schema Subdocument class