Fix using both `stdin: 'inherit'` and `input` options

[ehmicky]

See #1218.

When using any of the following options:

• {input: 'foo', stdio: 'inherit'}
• {input: 'foo', stdio: ['inherit', ...]}
• {input: 'foo', stdin: 'inherit'}

Execa should silently uses the following instead: {input: 'foo', stdin: 'pipe', ...}.

Please note that if users need multiple inputs, they can still use the {stdin: ['inherit', {string: 'foo'}] option.

This is technically a breaking change, so should be in a major release.