Deprecate execa.shell() in favor of execa(..., { shell: true })

[ehmicky]

Should we deprecate and/or remove execa.shell() in favor of execa(..., { shell: true })?

The reason being shell is a misused option, which creates problems, so we should not encourage its use by providing a shortcut method.

Some of the issues with the shell option:

1. It is not cross-platform. While it does call cmd.exe on Windows and /bin/sh on Unix, those shells have different syntaxes. If the command uses shell-specific syntax, it won't be cross-platform. It it does not, shell is not needed.
2. It is slower, as it interprets the command through a shell interpreter.
3. It is unsafe as it allows for command injection.
4. It is often used only to be able to pass command and arguments as a string string. Which is not an issue anymore since Support single string input without using the shell option #182

There are some legitimate use cases for the shell option, but I believe 99% of the times its usage does not satisfy the following criteria:
a) shell is only useful for shell-specific features not available in JavaScript and Node.js. There are really not many of those.
b) shell is only useful when cross-platform is not a concern.

Note: I can submit a PR if approved.

[sindresorhus]

👍 Also include your above points in the shell option docs and TS type for it.

[ehmicky]

Done in #219