Deprecating "features"

[emillon]

The "features" field is accepted in opam files:

https://github.com/ocaml/opam/blob/4d9db690420e36d995e6a0afe18660bcb45bea49/src/format/opamFile.mli#L457-L458

It is documented as an experiment but it looks like the feature was never used. Something similar is likely to be useful with package variables but it looks like this first implementation can be deprecated in 2.1 or 2.2.

[dra27]

See https://github.com/ocaml/opam/issues/4519#issuecomment-868445795

[AltGr]

This is different from libraries and syntax in that, although unused, it is documented and doesn't do absolutely nothing. I'd say we can keep it (with the experimental flag) until we have an actual implementation for package variables.