Allow pasting registers from QuickPick menu

[MGessinger]

What this PR does / why we need it:

This PR adds extends the :display command, so that the QuickPick which opens to show the registers also offers action buttons to paste the register content. This is equivalent to running :di, followed by :put with the selected register. While the functionality is not new per se, it makes the QuickPick of the registers slightly more useful.

In order to do this, most of the changes are within registers.ts but some small changes were made to the Register class as well. These are convenience functions, and they do not alter any existing functionality.

Which issue(s) this PR fixes

I couldn't find an issue which talks about this feature. If a linked issue is required, I can open one afterwards and discussion can happen there, but at least for me this feature would improve my productivity greatly.

Special notes for your reviewer:

I know it's a mouse action rather than a keyboard action. The Vim gods shall have mercy on my soul.