key: add ModCtrl, ModShortcut

ModCtrl is the physical Ctrl key, ModShortcut is the virtual
"shortcut" modifier, which is Ctrl on most platforms, Command on
Apple platforms.

Updates #59

Signed-off-by: Elias Naur <mail@eliasnaur.com>

io/key/mod.go

@@ -0,0 +1,9 @@
+// SPDX-License-Identifier: Unlicense OR MIT
+
+// +build !darwin
+
+package key
+
+// ModShortcut is the platform's shortcut modifier, usually the Ctrl
+// key. On Apple platforms it is the Cmd key.
+const ModShortcut = ModCtrl