feat: add ruby/bundler skeleton (#2852)

lib/config/definitions.js

lib/manager/bundler/artifacts.js

lib/manager/bundler/extract.js

lib/manager/bundler/index.js

lib/manager/bundler/range.js

lib/manager/bundler/readme.md

lib/manager/bundler/update.js

lib/manager/index.js

test/manager/__snapshots__/manager-docs.spec.js.snap

test/manager/newmanager/artifacts.spec.js

test/manager/newmanager/extract.spec.js

test/manager/newmanager/range.spec.js

test/manager/newmanager/update.spec.js

...rkers/repository/extract/__snapshots__/index.spec.js.snap