Typings improvement.

[MadProbe]

I want to suggest some improvement on typings:

1. Mark the length property as readonly since only getter is defined for this property.
2. I would use optional arguments syntax, e.g. function foo(argument?: whatever) {} instead on consequencial overloads since it's considered as a best practice. (https://github.com/invertase/denque/blob/master/index.d.ts#L4-L8)

[Salakar]

We're willing to accept a PR to improve the types if you're interested?

[MadProbe]

Sure, i'll open a PR in a couple of days.