Behaviors: Add 'ignore-modifiers' option to sticky keys

To combine multiple sticky modifiers, the sticky keys must ignore
other (sticky) modifier keypresses.

This behavior is important for "callum-style mods", where all modifiers
are sticky mods.

Fixes #829

app/dts/bindings/behaviors/zmk,behavior-sticky-key.yaml

@@ -15,3 +15,5 @@ properties:
     type: int
   quick-release:
     type: boolean
+  ignore-modifiers:
+    type: boolean