Create dev-middleware package (#38194)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/38194

## Context

RFC: Decoupling Flipper from React Native core: https://github.com/react-native-community/discussions-and-proposals/pull/641

## Changes

Inits the `react-native/dev-middleware` package. Contains an initial implementation of `/open-debugger`, migrated from https://github.com/react-native-community/cli/commit/2535dbe2346a390b2c5034acf0b348347fce0b73.

## 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