Remove execa.shell() and execa.shellSync()

[ehmicky]

Fixes #211

Since the next version is a major release, I thought this would be the perfect time to directly remove those methods instead of deprecating them.

[ehmicky]

Fixed according to the code review.

[ehmicky]

Fixed.

[sindresorhus]

The TS text should be exactly what's in the readme. Easier to keep them in sync that way.

[sindresorhus]

Can you fix the merge conflict?

[ehmicky]

Fixed

[kopax]

Too bad you are not showing any concret example of before/after of this breaking change.

[ehmicky]

Hi @kopax,

You might get some extra information in this Medium article.

Generally speaking:
Before: execa.shell(command) and execa.shellSync(command)
After: execa(command, {shell: true}) and execa.sync(command, {shell: true})

[kopax]

Thanks, I finally understood by myself, but that's always annoying to have to read the whole documentation to find how the breaking change is affecting our code. I recommend to always have a concrete migration example for all of them

[ehmicky]

The release notes show:

### Breaking changes
- Remove `execa.shell()` and `execa.shellSync()`. The [`shell option`](https://github.com/sindresorhus/execa#shell) should be used instead.

The shell option includes a link to its documentation, I think it should be a quite clear migration path. I agree that an explicit before/after example would have been better, but the release notes weren't too cryptic either.

[kopax]

I got through this but my advice is still valid, that's just general advice so people in general don't have to go through as I did.