Fix Record#get type checking

[bigmontz]

This method was not correctly type checking the key argument. Keys are not in the Entries where being accepted without trigger typescript errors.

The error was being cause because the method was not relying in the keys set in the Entries, but the ones came from the constructor and from the field lookup.

The keys came from constructor and field lookup are not meant to be used in the client code, since they are internal.

This way, the error was solving by strict get method key type for consider only indexes (number) and Key originated from the Entries.

Example:

interface Person {
   age: Integer
   name: string
}

const p: Record<Person> = // get record form somewhere

const age: Integer = p.get('age')
const name: string = p.get('name')

// @ts-expected-error This error was not being point out before
const nonExistingKey = p.get('non-existing-key')

⚠️ This type definitions are not asserted in runtime. Thus mismatched type records coming from the database will not trigger type errors.

[robsdedude]

🦐