Add support for rebinding Column Selection

[ChiriVulpes]

Currently there is no way to rebind column selection. The only bindings that can be changed for column selection are as follows:

In other programs it's bound by default to middle click or alt+drag, but in VSCode it's bound to alt+shift. It's not an inherently bad thing to do bindings different than other programs, but it'd be nice if it was configurable. I'd really like to change it to alt, personally. And I'm not alone, either.

It's possible that it can already be rebound, but as it's not in the Default Keyboard Shortcuts file, there's no way for the basic user to find out that it can be rebound. This would be an even easier fix if that was the case, tho--simply add an entry for it.

[alexdima]

👍 Let's track this request in #3130