[Keyboard] Add Janus keyboard (#18367)

Co-authored-by: Ryan <fauxpark@gmail.com>

keyboards/janus/keymaps/default/keymap.c

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+// Copyright 2022 Steven Karrmann (@skarrmann)
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+#include QMK_KEYBOARD_H
+
+enum layer_names {
+    _BASE,
+    _SYMBOL,
+    _NAVIGATION,
+    _FUNCTION
+};
+
+#define MO_SYM MO(_SYMBOL)
+#define MO_NAV MO(_NAVIGATION)
+#define MO_FUN MO(_FUNCTION)
+#define OS_LCTL OSM(MOD_LCTL)
+#define OS_LSFT OSM(MOD_LSFT)
+#define OS_LALT OSM(MOD_LALT)
+#define OS_LGUI OSM(MOD_LGUI)
+#define OS_RCTL OSM(MOD_RCTL)
+#define OS_RSFT OSM(MOD_RSFT)
+#define OS_RALT OSM(MOD_RALT)
+#define OS_RGUI OSM(MOD_RGUI)
+
+const uint16_t PROGMEM keymaps[][MATRIX_ROWS][MATRIX_COLS] = {
+    [_BASE] = LAYOUT_split_3x5_2(
+        KC_Q   , KC_W   , KC_E   , KC_R   , KC_T   , KC_Y   , KC_U   , KC_I   , KC_O   , KC_P   ,
+        KC_A   , KC_S   , KC_D   , KC_F   , KC_G   , KC_H   , KC_J   , KC_K   , KC_L   , KC_QUOT,
+        KC_Z   , KC_X   , KC_C   , KC_V   , KC_B   , KC_N   , KC_M   , KC_COMM, KC_DOT , KC_SLSH,
+                                   MO_NAV , KC_LSFT, KC_SPC , MO_SYM
+    ),
+    [_SYMBOL] = LAYOUT_split_3x5_2(
+        KC_SCLN, KC_LBRC, KC_RBRC, KC_5   , XXXXXXX, XXXXXXX, KC_6   , KC_MINS, KC_EQL , KC_GRV ,
+        KC_1   , KC_2   , KC_3   , KC_4   , KC_BSLS, XXXXXXX, KC_7   , KC_8   , KC_9   , KC_0   ,
+        OS_LGUI, OS_LALT, OS_LSFT, OS_LCTL, XXXXXXX, XXXXXXX, OS_RCTL, OS_RSFT, OS_RALT, OS_RGUI,
+                                   MO_FUN , _______, _______, _______
+    ),
+    [_NAVIGATION] = LAYOUT_split_3x5_2(
+        KC_PSCR, KC_INS , KC_APP , KC_DEL , XXXXXXX, XXXXXXX, KC_HOME, KC_PGDN, KC_PGUP, KC_END ,
+        KC_ESC , KC_TAB , KC_ENT , KC_BSPC, XXXXXXX, XXXXXXX, KC_LEFT, KC_DOWN, KC_UP  , KC_RGHT,
+        OS_LGUI, OS_LALT, OS_LSFT, OS_LCTL, XXXXXXX, XXXXXXX, OS_RCTL, OS_RSFT, OS_RALT, OS_RGUI,
+                                   _______, _______, _______, MO_FUN
+    ),
+    [_FUNCTION] = LAYOUT_split_3x5_2(
+        KC_F1  , KC_F2  , KC_F3  , KC_F4  , QK_BOOT, KC_SLCK, KC_F9  , KC_F10 , KC_F11 , KC_F12 ,
+        KC_F5  , KC_F6  , KC_F7  , KC_F8  , XXXXXXX, KC_CLCK, KC_MUTE, KC_VOLD, KC_VOLU, KC_PAUS,
+        OS_LGUI, OS_LALT, OS_LSFT, OS_LCTL, XXXXXXX, KC_NLCK, OS_RCTL, OS_RSFT, OS_RALT, OS_RGUI,
+                                   _______, _______, _______, _______
+    ),
+};

keyboards/janus/keymaps/default/readme.md

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+# Janus Default Keymap
+
+![Janus default keymap image](https://i.imgur.com/7FevUIZh.png)
+
+The default Janus keymap is simple and organized. The only QMK special features used are layers and one shot keys for modifiers.
+
+It is inspired by [Callum's keymap](https://github.com/qmk/qmk_firmware/tree/master/users/callum), with a few differences:
+
+* Modifier keys are moved to the bottom row on every layer, and are mirrored on both sides.
+* Symbol layer only contains non-shifted symbol keys.
+* General differences in key positions.
+
+For simplicity's sake, the keymap uses QMK's native one shot key implementation. There are other excellent one shot implementations out there, including Callum's in the aforementioned keymap.