feat: Add the `WithUsage` function.

[octo]

Add a wrapper for PositionalArgs that supplements errors with usage information.

The error messages provided by the PositionalArgs functions is quite terse, e.g.

accepts n arg(s), received m

In many cases this will not be enough information for users to correct their mistake. The new WithUsage function includes the usage string in the error message to improve the user experience.

Example:

cmd := &cobra.Command{
	Use:  "example <arg>",
	Args: cobra.WithUsage(cobra.ExactArgs(1)),
}

[CLAassistant]

All committers have signed the CLA.

[marckhouzam]

I like this. And I like the use of an example function in the tests, which we haven’t done before. Nice job.