esy/pjs/cross language support progress

[bestander]

This ticket tracks the progress of making Yarn work with native languages.

• PJS spec implementation draft [WIP] Implement PJC spec #2792
• esy spec https://github.com/esy-ocaml/esy
• package.json dynamic dependencies Feature Request: Variable Substitution in package.json #1380

[Daniel15]

@torifat had a proof-of-concept/WIP diff for supporting Composer packages too.

[torifat]

It's here -> https://github.com/torifat/yarn/tree/feature/packagist