yarn v4 support

[lizthegrey]

Is your enhancement request related to a problem? Please describe. While the new prune plugin works great on yarn 2/3, it fails on yarn 4:

       Running 'yarn heroku prune'
       Type Error: r.Command.Path is not a function (when initializing @yarnpkg/plugin-prune-dev-dependencies, defined in <environment>)
       at /tmp/buildpack9a0bK/yarn2-plugins/prune-dev-dependencies/bundles/@yarnpkg/plugin-prune-dev-dependencies.js:5:1353
       at factory (/tmp/buildpack9a0bK/yarn2-plugins/prune-dev-dependencies/bundles/@yarnpkg/plugin-prune-dev-dependencies.js:5:1438)
       at ve (/app/.yarn/releases/yarn-4.0.0-rc.6.cjs:135:21634)
       at Rm (/app/.yarn/releases/yarn-4.0.0-rc.6.cjs:134:30693)
       at ee (/app/.yarn/releases/yarn-4.0.0-rc.6.cjs:135:21614)
       at Function.find (/app/.yarn/releases/yarn-4.0.0-rc.6.cjs:135:21835)
       at async s (/app/.yarn/releases/yarn-4.0.0-rc.6.cjs:378:2549)
       at async r (/app/.yarn/releases/yarn-4.0.0-rc.6.cjs:378:2152)

Describe the solution you'd like The module will need to be updated to use the updated Yarn plugin API.