Remove fs.readFile from some plugins

Summary:
1. Remove fs.readFile from some plugins
2. Add "importFile" to FlipperLib and RenderHost

See D32492149 for context

Followups:

1. Decapitate Stella's sendToPhone
2. Decapitate crash reporter
3. Figure out how to approach navigation
4. Figure out how to approach FileSelector

Reviewed By: mweststrate

Differential Revision: D32496775

fbshipit-source-id: e150aa56a2c2c1eb12a4c03c801f76cd76485a9d

desktop/flipper-plugin/src/index.ts

@@ -37,6 +37,7 @@ export {
   FlipperLib,
   getFlipperLib,
   setFlipperLibImplementation as _setFlipperLibImplementation,
+  FileDescriptor,
 } from './plugin/FlipperLib';
 export {
   MenuEntry,