refactor: Promote debounce to exposed mod lib.

* Promote previously local debounce code from kscan drivers to exposed
  module lib, for use with other drivers as needed.
* Refactor existing kscan driver to new "public" API.

app/module/include/zmk/debounce.h

@@ -0,0 +1,56 @@
+/*
+ * Copyright (c) 2021 The ZMK Contributors
+ *
+ * SPDX-License-Identifier: MIT
+ */
+
+#pragma once
+
+#include <stdbool.h>
+#include <stdint.h>
+#include <zephyr/sys/util.h>
+
+#define DEBOUNCE_COUNTER_BITS 14
+#define DEBOUNCE_COUNTER_MAX BIT_MASK(DEBOUNCE_COUNTER_BITS)
+
+struct zmk_debounce_state {
+    bool pressed : 1;
+    bool changed : 1;
+    uint16_t counter : DEBOUNCE_COUNTER_BITS;
+};
+
+struct zmk_debounce_config {
+    /** Duration a switch must be pressed to latch as pressed. */
+    uint32_t debounce_press_ms;
+    /** Duration a switch must be released to latch as released. */
+    uint32_t debounce_release_ms;
+};
+
+/**
+ * Debounces one switch.
+ *
+ * @param state The state for the switch to debounce.
+ * @param active Is the switch currently pressed?
+ * @param elapsed_ms Time elapsed since the previous update in milliseconds.
+ * @param config Debounce settings.
+ */
+void zmk_debounce_update(struct zmk_debounce_state *state, const bool active, const int elapsed_ms,
+                         const struct zmk_debounce_config *config);
+
+/**
+ * @returns whether the switch is either latched as pressed or it is potentially
+ * pressed but the debouncer has not yet made a decision. If this returns true,
+ * the kscan driver should continue to poll quickly.
+ */
+bool zmk_debounce_is_active(const struct zmk_debounce_state *state);
+
+/**
+ * @returns whether the switch is latched as pressed.
+ */
+bool zmk_debounce_is_pressed(const struct zmk_debounce_state *state);
+
+/**
+ * @returns whether the pressed state of the switch changed in the last call to
+ * debounce_update.
+ */
+bool zmk_debounce_get_changed(const struct zmk_debounce_state *state);