Experiments: Add an extension Command to stop a running experiment

[rogermparent]

Currently, we run dvc exp run via IntegratedTerminal, so you can ctrl+c in the Terminal to cancel a running experiment.

This isn't optimal for a few reasons:

• If we switch away from IntegratedTerminal (which is likely) this will stop working
• It's just plain unfriendly to users, and we can do better with at least a command

Since we can send text to even the IntegratedTerminal, we should be able to implement a command that sends ^C to the currently tracked terminal and expose a cancel feature to the VSCode command UI.

We'll also need to access this from some points of the UI, and probably the Webview too. #231

I think it would be best to use this Issue to track the command specifically, and then open new specific tickets for the Webview and other UI additions.