Implement completion provider for #Requires statements

[rkeithhill]

Not sure if this used to work but it isn't in 1.2.1 and it would be really nice to have.

[daviwil]

Doesn't work because the PowerShell engine doesn't give completions for it. However, we should be able to start adding custom completions soon when I add the completions provider APIs :)

[rkeithhill]

Cool. I'd like to write this one.

[daviwil]

That'd be great!

[daviwil]

Depends on this issue: #494