[Core] Add Repeat Key ("repeat last key") as a core feature. (#19700)

Co-authored-by: casuanoob <96005765+casuanoob@users.noreply.github.com>
Co-authored-by: Sergey Vlasov <sigprof@gmail.com>

quantum/process_keycode/process_repeat_key.h

@@ -0,0 +1,62 @@
+// Copyright 2022-2023 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#pragma once
+
+#include "quantum.h"
+
+/**
+ * @brief Process handler for remembering the last key.
+ *
+ * @param keycode  Keycode registered by matrix press, per keymap
+ * @param record   keyrecord_t structure
+ * @return true    Continue processing keycodes, and send to host
+ * @return false   Stop processing keycodes, and don't send to host
+ */
+bool process_last_key(uint16_t keycode, keyrecord_t* record);
+
+/**
+ * @brief Optional callback defining which keys are remembered.
+ *
+ * @param keycode          Keycode that was just pressed
+ * @param record           keyrecord_t structure
+ * @param remembered_mods  Mods that will be remembered with this key
+ * @return true            Key is remembered
+ * @return false           Key is ignored
+ *
+ * Modifier and layer switch keys are always ignored. For all other keys, this
+ * callback is called on every key press. Returning true means that the key is
+ * remembered, false means it is ignored. By default, all non-modifier,
+ * non-layer switch keys are remembered.
+ *
+ * The `remembered_mods` arg represents the mods that will be remembered with
+ * this key. It can be modified to forget certain mods, for instance to forget
+ * capitalization when repeating shifted letters:
+ *
+ *     // Forget Shift on letter keys.
+ *     if (KC_A <= keycode && keycode <= KC_Z && (*remembered_mods & ~MOD_MASK_SHIFT) == 0) {
+ *         *remembered_mods = 0;
+ *     }
+ */
+bool remember_last_key_user(uint16_t keycode, keyrecord_t* record, uint8_t* remembered_mods);
+
+/**
+ * @brief Process handler for Repeat Key feature.
+ *
+ * @param keycode  Keycode registered by matrix press, per keymap
+ * @param record   keyrecord_t structure
+ * @return true    Continue processing keycodes, and send to host
+ * @return false   Stop processing keycodes, and don't send to host
+ */
+bool process_repeat_key(uint16_t keycode, keyrecord_t* record);