Fix timeout option validation

[ehmicky]

Before Node 15.13.0 (link to PR), the timeout was only available in execFile() and exec() but not in spawn(), which is used by Execa.

Execa implemented its own timeout option to fix this. That option probably won't be needed after support for Node 15.13.0 is dropped.

In the meantime, the timeout option added to child_process.spawn() introduces the following inconsistent behavior in Execa depending on the Node.js version:

• In Node <15.13.0, using an invalid timeout (such as a negative integer) will throw an exception, using Execa's validation logic
• In Node >=15.13.0, it will return a rejected promise instead, using child_process.spawn() validation logic

This PR fixes this inconsistency by moving Execa's validation logic earlier, before child_process.spawn() is called.

CI tests are failing due to #462.

[sindresorhus]

Can you fix the conflict?

[ehmicky]

Fixed 👍

Some CI tests are failing due to #464 (unrelated to this PR).