Add deprecation for global installs

Created by: mrmckeb

Is your proposal related to a problem?

When rolling out new features that require changes to the create-react-app package, we regularly see issues related to users with older, global installs of that package.

Describe the solution you'd like

Ideally, we should add a deprecation warning in react-scripts so that when it a user attempts to install it with a globally installed create-react-app, they're asked to remove it and use npx or yarn create instead.

Describe alternatives you've considered

Alternatively, or additionally, we could add this to create-react-app itself, but that won't fix the issue for those with older installs.

Additional context

This came out of a discussion with @iansu.

Created by: miraage

Could you please clarify what console commands should raise errors?

Created by: mrmckeb

@miraage, something like:

Global installs of create-react-app are now deprecated. Please remove this package with [command], and instead use npx or yarn.

We'd either include the updated commend(s), or provide a link.

Created by: sagarnasit

Hey @mrmckeb, I would like to contribute with this good first issue. Can you guide me on how to start with this?

Created by: mrmckeb

Hi @sagarnasit, of course.

Let's ignore the exact messaging now and get a basic PR up, and then we can tune it.

First, we need to detect if the package was installed globally:

• We can do this via a script
• We can do this by looking at the package path and seeing if it is in the global path - npm config get prefix
• There's an implementation to reference here too - but I'm not sure if it's the right solution.

The files we'll be doing this in are:

• https://github.com/facebook/create-react-app/blob/master/packages/create-react-app/createReactApp.js (you can see we already have warnings in here, like for react-scripts-ts).
• https://github.com/facebook/create-react-app/blob/master/packages/react-scripts/scripts/init.js

So, whenever either of those scripts is run and create-react-app is globally installed, we need to give a warning that global installs are deprecated.

Let me know if that helps (or doesn't) :)