(More) TypeScript polish

[Timer]

• don't resolve .ts|tsx extensions without typescript detected: Automatically setup TypeScript when detected #5549
• add --typescript to create-react-app cli Add TypeScript app creation #5550
• don't allow launch when .ts files detected but there's no TypeScript Automatically setup TypeScript when detected #5549

[miraage]

What's the purpose for the --typescript flag? Is it up to grabs?

[Timer]

See #5550