Deprecating old `ipc` option

[ehmicky]

We've replaced the {stdio: [..., 'ipc']} option with the simpler {ipc: true}. The old syntax has been undocumented. However, it's still currently supported, for backward compatibility.

In the next major release, should we remove support the old syntax? Possible options:

1. Keep as is: undocumented only
2. Remove the types only
3. Remove the types, and also throw an error message when using that old syntax at runtime

Which one do you prefer @sindresorhus?

Note: we probably should wait for many months before doing any new major release. We just did a big major release with many breaking changes. So this issue is for future planning, not immediate action.

[sindresorhus]

I would do 3.