improve type definition of from(obj: object)

[Neuroboy23]

Prior to this change, using Enumerable.from(...) on a simple map object with typed keys would lose the key type. This may be intentional due to edge cases with which I am unfamiliar. I am submitting this PR in hopes that is not the case.

type KeyType = "foo" | "bar" | "baz";

const input: Record<KeyType, number> = {
    foo: 1,
    bar: 2,
    baz: 3
};

const output = Enumerable.from(input)
    .toObject(
        e => e.key,
        e => e.value * 10
    );
    
// result:  output is a `Record<string, number>`

// desired: output is a `Record<KeyType, number>`