Migrate Alias's config from a list of entries to a `name->alias_data` mapping

### What component of Red (cog, command, API) would you like to see improvements on?

Alias

### Describe the enhancement you're suggesting.

The current data structure seems inefficient. This isn't much of an issue when calling an alias since we maintain a cache in the cog but it is not ideal for edits or deletions which require finding the alias in the entries list. Using a custom group instead would be preferred.

Low-priority issue but would IMO be nice to change this at some point.

### Anything else?

_No response_

Start by locating the Alias cog's configuration entries and its cache, then trace how edits and deletions find aliases in the list. Review the existing custom-group support and determine the migration path for the stored data. Done means Alias configuration uses a name-to-alias_data mapping while calling, editing, and deleting aliases continue to work.

Written by the indexing model from the issue text.