Create dev-middleware package

Summary:
X-link: facebook/react-native#38194

## Context

RFC: Decoupling Flipper from React Native core: react-native-community/discussions-and-proposals#641

## Changes

Inits the `react-native/dev-middleware` package. Contains an initial implementation of `/open-debugger`, migrated from react-native-community/cli@2535dbe.

## Attribution

This implementation is greatly inspired by `expo/dev-server`: https://github.com/expo/expo/blob/1120c716f35cb28d88800e8f5d963d2b2ac94705/packages/%40expo/dev-server/src/JsInspector.ts#L18

Changelog: [Internal]

Reviewed By: motiz88

Differential Revision: D46283818

fbshipit-source-id: 7b38ad2f6d7346366a7c599d16e289e04b7bd88d

Author: huntie

flow-typed/npm/connect_v3.x.x.js

@@ -21,8 +21,7 @@ declare module 'connect' {
     res: http.ServerResponse,
     next: NextFunction,
   ) => void | Promise<void>;
-  declare export type HandleFunction =
-    | NextHandleFunction;
+  declare export type HandleFunction = NextHandleFunction;
 
   declare export interface IncomingMessage extends http.IncomingMessage {
     originalUrl?: http.IncomingMessage['url'];