Enable a way to enforce the length of a `ParticleList` upon creation

Feature description

We should create a mechanism by which we can easily enforce the length of a ParticleList.

Motivation

When dealing with particle interactions, it's common to need to create a ParticleList of length two. Right now, it's necessary to add a couple of lines of code:

if len(particle_list) != 2:
    raise ValueError("Add an error message here.")

This isn't so long, but we'd need to recreate the error message each time, so we'd end up with repeating code. Given how frequently this comes up, it'd be helpful to have a more standard error message and way of handling this situation.

Implementation strategy

This could be in ParticleList itself, via @particle_input(), or through the creation of a separate function.

If we do this via @particle_input(), we could use the new annotated typing capabilities. These are the capabilities that let us have annotations like u.Quantity[u.kg], and use the __class_getitem__ dunder, so it could be ParticleListLike[2]. Alternately, we could have a separate keyword argument in the signature of @particle_input().

When the length is

Additional context

Many thanks to @ejohnson-96 for originally discussing this idea with me!

Start by reading the ParticleList implementation and the @particle_input() entry point, since the issue proposes either location for enforcing length. Compare the possible approaches described, then define the behavior and standard error message for length-constrained ParticleList creation; the issue does not name a test file.