Reorganise dev tooling/debugging modules as src/private/devsupport/ (#50730)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/50730

This is a consolidation of our "dev support" features in the `src/private/` dir.

New grouping:

- `src/private/devsupport/`
    - `devmenu/` — The Dev Menu and in-app inspection features (in-app UI)
        - `elementinspector/` — The Element Inspector overlay (panel with sub-features), accessed from  Dev Menu → Toggle Element Inspector
        - `perfmonitor/` — The Perf Monitor overlay, accessed from Dev Menu → Show Perf Monitor
    - `rndevtools/` — Modules supporting React Native DevTools (external UI)

`NativeReactDevToolsRuntimeSettingsModule.js` and `NativeReactDevToolsSettingsManager.js` are also moved out of `specs_DEPRECATED/`.

Changelog: [Internal]

Reviewed By: hoxyq

Differential Revision: D73031559

fbshipit-source-id: 952a469f31f4cd05aeed27dc7023552e723ca078