Provide a setting to configure the multi selection modifier key

[egamma]

With the new multi selection support a CTRL-Click no longer opens an editor to the side, but adds the selected item to the selection. Opening an editor to the side is now done using ALT-Click.

We should support a backward compatibility setting for users that prefer the old CTRL-Click to open an editor to the side. This is similar to the editor.multiCursorModifier.

[bpasero]

I added a new setting "workbench.multiSelectModifier": "alt" | "ctrlCmd" which controls the key to be used to add to the selection (if multi select is supported). The default will be ctrlCmd, but when you set it to alt you get back to be able to use ctrlCmd to “open to side”

@joaomoreno I had to touch the List widget for this a little bit because the multi-select handling is within the widget. I am allowing to set a new IListOptions.useAltAsMultiSelectModifier property down to the mouse controller to control this.

@isidorn fyi

To verify:
Verify in the "Open Editors" list and the "Files Explorer" tree. By default you can use Ctrl/Cmd to add items to the multi-selection and Alt to "Open to Side". If you flip the setting, you can use Alt to add items to the multi-selection and Ctrl/Cmd to "Open to Side".