types(schema): allow calling schema.static() with as TStatics

[vkarpov15]

Re: #15780

Summary

My suggested workaround for #15780 would be to use as DocStatics, but that currently fails with the following error:

Type 'DocStatics' is not assignable to type '{ [name: string]: (this: Model<IDoc, {}, {}, {}, Document<unknown, {}, IDoc, {}, {}> & IDoc & Required<{ _id: unknown; }> & { __v: number; }, any>, ...args: any[]) => any; }'. Index signature for type 'string' is missing in type 'DocStatics'.

This PR adds an override to Schema.prototype.static() to support calling static() with exactly the TStatics type

Examples

[hasezoey]

LGTM