flipper-pkg template will now use sandy

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

desktop/pkg/templates/plugin/babel.config.js.template

@@ -0,0 +1,7 @@
+module.exports = {
+    presets: [
+        '@babel/preset-typescript', 
+        '@babel/preset-react', 
+        ['@babel/preset-env', {targets: {node: 'current'}}]
+    ],
+};