[Typings] Incorrect `readonly` annotations

[artemiswkearney]

Describe the bug

The Typescript typings for Fuse claim that the constructor argument for the initial collection (possibly other arguments elsewhere in the API) takes a ReadonlyArray, but the add, remove, and removeAt methods modify the array, which isn't legal with a ReadonlyArray

Version

v6.6.2

Is this a regression?

No

🔬Minimal Reproduction

1. Create an array with some documents in it.
2. Put a reference to that array in a variable of type ReadonlyArray<T> or readonly T[], guaranteeing to the type system that that reference can't be used to modify the array.
3. Construct a Fuse instance from that variable.
4. Modify the Fuse instance's collection by calling the add, remove, and/or removeAt methods.
5. The original array has now been modified through what was supposed to be a readonly reference.

Additional context

https://www.typescriptlang.org/docs/handbook/2/objects.html#the-readonlyarray-type