Add deprecated as an alias for deprecate

[bernardnormier]

Currently the metadata for a deprecated definition in Slice is "deprecate" and not the more common "deprecated".

I propose to keep "deprecate" but add "deprecated" as an alias. This way, you can use either and the result in the same.

[InsertCreativityHere]

I'd be fine with adding this, although I'm not sure how much value it'll provide.

[InsertCreativityHere]

So, we aren't even consistent ourselves. The metadata you use to mark something deprecated is deprecate But the string you pass in to ignore deprecation warnings is deprecated

Should we also let suppress-warning support deprecate?

[bernardnormier]

No, we should standardize on deprecated, not add more support for 'deprecate'.

[InsertCreativityHere]

This was implemented in #2166.