Summary:
This diff updates plugin scaffolding by using Sandy and closely following the internal Scarf templates (see D24949452, D24949452)
By using `flipper-plugin`, it is now also possible to write unit tests for a plugin, and the default infra for that is generated (babel / jest)
For now there is still a dependency on `flipper` to support fancy components not yet available in Sandy, this will be updated in the future: T79632585
Changelog: `flipper-pkg init` now uses the new Sandy plugin infrastructure ant Ant.design component system
Reviewed By: nikoant
Differential Revision: D24950080
fbshipit-source-id: afc5e7ac728b20cb84fdbbdcb76cd45968736c01
dc82ec2885