refactor(combos): Reduce RAM usage, simplify config (#2849)

* Reference combos by index, not 32-bit pointers, and store bitfields
instead of arrays in several places, to bring down our flash/RAM usage.
* Use bit field to track candidate combos, to avoid needing an explicit
`ZMK_COMBO_MAX_COMBOS_PER_KEY` setting.
* Determine the max keys per combo automatically from the devicetree,
so we remove the ZMK_COMBO_MAX_KEYS_PER_COMBO Kconfig symbol.

app/dts/bindings/zmk,combos.yaml

@@ -25,4 +25,3 @@ child-binding:
       type: boolean
     layers:
       type: array
-      default: [-1]