Implement `SystemParam` for `SmallVec` (#24405)

# Objective

To register a system from a script, I need to support for dynamic number
of system parameters (since scripting languages usually don't play well
with generics). Right now the only solution is to use `Vec<T>` as a
system param which is allocated every time the system runs. But it would
be nice to avoid allocations when the number of system parameters is
low.

## Solution

Implement `SystemParam` for `SmallVec`. It's already a dependency for
`bevy_ecs` and allows to avoid allocations with user-defined number of
system parameters.

## Testing

- Did you test these changes? If so, how? I tried creating a system with
a `SmallVec<[Local<usize>; 8]>`.
- Are there any parts that need more testing? I don't think so. The code
follows `Vec` impl.