Migration refactorings after renaming from `.js` to `.ts` files

At least week's design meeting, @sheetalkamat had the idea that we should consider applying refactorings for when you rename a `.js` file to a `.ts` file. I think this is definitely worth experimenting with. From the refactoring pipeline, I could imagine things like * moving from CommonJS to ES Modules * lifting JSDoc annotations * moving to classes * inferring from usages Then there's project configuration. I could see * Generating a `tsconfig.json` * Installing `@types` dependencies for packages that need them. @mjbvz @amcasey