[Keymap] major keymap overhaul (#10185)

* experiment with userspace

* reorganise

* readme

* missing oneshot shift from ignored keys

* recombine hands in layout macro

users/callum/swapper.h

@@ -0,0 +1,20 @@
+#pragma once
+
+#include QMK_KEYBOARD_H
+
+// Implements cmd-tab like behaviour on a single key. On first tap of trigger
+// cmdish is held and tabish is tapped -- cmdish then remains held until some
+// other key is hit or released. For example:
+//
+//     trigger, trigger, a -> cmd down, tab, tab, cmd up, a
+//     nav down, trigger, nav up -> nav down, cmd down, tab, cmd up, nav up
+//
+// This behaviour is useful for more than just cmd-tab, hence: cmdish, tabish.
+void update_swapper(
+    bool *active,
+    uint16_t cmdish,
+    uint16_t tabish,
+    uint16_t trigger,
+    uint16_t keycode,
+    keyrecord_t *record
+);