Demote `electron` to devDependency (#53438)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/53438

Changelog: [Internal]

Makes `flavor: 'prebuilt'` the default mode of launching the RNDT standalone shell, and the *only* mode supported in the published version of the package. See D78351931 for more context.

With this, we can demote `electron` from `dependencies` to `devDependencies`. This makes it possible to make `debugger-shell` a dependency of `dev-middleware` (and thus of all major frameworks) without significantly impacting `npm install` times. We'll add this dependency on `debugger-shell` in an upcoming diff (D78351937).

We also stop publishing the `dist/electron` subdirectory (and `src/electron` for good measure) since the corresponding code will always be bundled into the prebuilt binary instead.

Reviewed By: huntie

Differential Revision: D78351934

fbshipit-source-id: 2a4b03e852c4d0330250567c41dca09d1c4f3abd