[alias] attribute

PR #2344 added a new attribute, `[alias]` providing an alternative way for declaring recipe aliases. I personally lean towards not adding a new attribute, but I don't feel super strongly, so if it's something that people wanted, I wouldn't mind adding it.

I think ideally, would take a string literal: `[alias: "foo"]`, which would initially be constrained to be a valid identifier. This could be relaxed in the future to allow non-identifier aliases.

See the original PR for some of the discussion.