Allow F12 to be bound to both go to definition and open link

I tried to get this working with this but I don't think we have a context key for this: ``` { "key": "f12", "command": "editor.action.openLink", "when": "!jumpToCursorSupported"} ``` I think ideally this is what I'm after: ``` { "key": "f12", "command": "editor.action.openLink", "when": "linkAtCursor"} ``` I've been after this for a while as it would make using markdown much nicer imo. cc @lostintangent (tried to get it working for wikilens)