Implement `Reflect` for `Box<[T]>`

## What problem does this solve or what need does it fill?

For immutable arrays with variable runtime len, it's idiomatic to use `Box<[T]>` type.

## What solution would you like?

Similarly to an array `[T; N]`, I should be able to derive `Reflect` on types that contain `Box<[T]>`.
I'd send an MR with this feature, except I was not sure whether this type fit better `TypeInfo::List` or `TypeInfo::Array`.
The issue with the former is that there musn't be any appends, so cannot implement `List` due to required `insert` and `remove.
The issue with the latter is that `Typed::type_info` requires the capacity to be known without any reference to self.

## What alternative(s) have you considered?

I could use `Vec`, but I'd prefer `Box<[T]>`.

Start by tracing the existing Reflect support for [T; N], Box, and Vec, then inspect TypeInfo::List, TypeInfo::Array, and Typed::type_info. Decide which representation fits Box<[T]> without permitting append or remove operations, and verify that types containing it can derive Reflect.

Written by the indexing model from the issue text.