[maintainer] refactor(app-shell): move Studio's flow rail helpers, pillar kit and Data pillar helpers out of StudioDesignSurface.tsx (slice 4, final: file under 10,000 lines)

[maintainer] refactor(app-shell): move Studio's flow rail helpers, pillar kit and Data pillar helpers out of StudioDesignSurface.tsx (slice 4, final: file under 10,000 lines) #5882