Vertical split shortcut keys

[beefsack]

This PR binds C-w v to workbench.action.splitEditor, achieving similar functionality to what happens in Vim.

I've previously manually maintained a shortcut to match the Vim vertical split shortcut, but thought it might be helpful to add it for everyone else. Please close if it's not a complete fit, I'm happy to maintain the shortcut configuration myself.

At the moment it's just the binding, had a look at the automated tests and not entirely sure how I'd go about testing it. If this is required, some rough direction would be greatly appreciated.

Keep up the great work, this is one of the best Vim modes I've ever used, if not the best.

Relates to #1451.

[Chillee]

These look pretty good to me!

You're right that we don't have automated tests for these... It seems pretty difficult and not very helpful to do something like that.

Thanks for the PR! And thanks for the praise!