Feature idea/improvement: allow dev server and build script to run in presence of TypeScript errors

Created by: kylebebak

This isn't a bug report, but rather an idea to make CRA's TypeScript setup more user-friendly, and more aligned with what I understand to be the "spirit" of TypeScript.

Currently, for projects built with npx create-react-app <app> --typescript, or migrated to TypeScript as per the docs, both npm start and npm build fail in the presence of TypeScript type errors.

The dev server refuses to serve the app if non-fatal type errors are present. The build script builds it just fine, but exits with a non-zero status. In other words, npm build outputs valid code to /build that can deployed to a production server, even as it tells the user that the app "Failed to compile".

Devs migrating a moderately sized CRA app to TypeScript may literally be faced with thousands of type errors initially. None of these prevents the app from running, either locally or in production, but devs have to correct or ignore every last error (not that easy, given the limitations of @ts-ignore) before they can continue developing the app.

IMO this goes against TypeScript's philosophy of making the migration path from JS -> TS as easy as possible, and allowing devs to "progressively" add type annotations to their code.

TypeScript devs already have many options at their disposal to prevent deployment of code that has type errors, if they want to impose this constraint on themselves. CRA definitely shouldn't impose it on them.

At the very least, I think there should be an option to enable or disable this constraint. I understand that CRA's philosophy is to avoid config wherever possible. If adding an env var, e.g. TSC_COMPILE_ON_ERROR, is not desirable because it means more config, then I think the default behavior should be for CRA to function normally in the presence of type errors, while printing them as warnings to the terminal/browser console.

I opened a PR with changes that do exactly this. Looking forward to hear what people think!