Rename `yarn start` to `yarn dev` and `yarn start:prod` to `yarn start` (#26209)

The `start` convention is a CRA convention but nobody else of the modern
frameworks / tools use this convention for a file watcher and dev mode.
Instead the common convention is `dev`. Instead `start` is for running a
production build that's already been built.

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Co-authored-by: Sebastian Silbermann <silbermann.sebastian@gmail.com>