Make "attach" only a QuickAccess thing, not a QuickPick thing (#304521)

* Make "attach" only a QuickAccess thing, not a QuickPick thing

I think there's room for improvement here - like, I think I would prefer for `attach` to be folded in to `accept`.

So this behavior of:
> The picker will close automatically before running this.

for accept, feels wrong and should be re-evaluated.

* Remove attachment handling from AbstractGotoSymbolQuickAccessProvider

* have attaching context only fire on ctrl/cmd

* Have shift be a keymod & adopt it in quick access

* tests