This commit changes how Remora.Discord.Commands maps slash commands,
enabling support for arrays to be parsed and accepted, despite Discord
STILL not having support for this.

For better or worse, a snippet of code had to be removed:

```cs
var collectionType = parameter.GetActualParameterType();

if (!collectionType.IsCollection() && !collectionType.IsArray)
{
    // Sanity check
    throw new InvalidOperationException("Collection parameter was not a collection.");
}
```
^ For some reason, `.IsCollection()` returns...false? I gave up on
trying to figure out why that was the case - the code still works fine
and there's other validity checks elsewhere.

BREAKING-CHANGE: Commands with the structure of `name__numbers`
(e.g. `param__1`) are *no longer valid* and will get erased by the
pre-parsing step that is a requisite to map arrays. This should be an
absolutely niche case, but is still technically a breaking change, and
should be documented as such.

Some other things of note:

- Attributes applied to the array apply to all elements in the array
 (this should go without saying, but hey)
- `[Range]` controls the array's length; `Min` = required, `Max` = Length
 - If Min < Max, `Min` elements will be required and `Max - Min` elements
   will be optional. Max will coallecse to Min when unspecified.
- Due to Discord's..questionable limitations, optional array parameters
  are placed at the end of the argument list for slash commands, but do
  preserve the order they're declared in relative to each other.

…ents' into velvet/feat/variadic-slash-arguments