[12.x] Types: EnumeratesValues Sum

[liamduckett]

Hey, this PR adds a more specific typehint to the return value of EnumeratesValues::sum.

$amount = $transactions->sum(fn (Transaction $transaction) => $transaction->amount);

// Currently: mixed
// After PR:  float

After implementing, I discovered that this doesn't work when the return type is literal. For example:

$amount = $transactions->sum(fn (Transaction $transaction) => 5);

// Currently: mixed
// After PR:  5
// Actual:    int

I don't believe this is a big deal for two reasons:

1. Passing a callable with a literal return type to sum doesn't feel realistic.
2. This issue already exists in EnumeratesValues::reduce (see below snippet)

$collection->reduce(fn(int $int, Transaction $transaction, int $key) => 5, 3)

// Currently: 5
// Actual:    3|5

Open to other ideas, that would fix my use case without breaking this (albeit seemingly strange) other one!

[AJenbo]

Nice work, this removed the need for a bunch of workarounds

[liamduckett]

Thanks @AJenbo! 😃