Allow invert of SPLIT_HAND_PIN logic (#13433)

* added support for inverting the hand pin for split keyboards

* Added docs about SPLIT_HAND_LOW_IS_LEFT

* Update docs/feature_split_keyboard.md

bring #define for split hand pin low for left half name in line with grid pin define

Co-authored-by: Joel Challis <git@zvecr.com>

* Update quantum/split_common/split_util.c

update split hand pin low is left name to match split hand grid define

Co-authored-by: Joel Challis <git@zvecr.com>

Co-authored-by: Joel Challis <git@zvecr.com>

docs/feature_split_keyboard.md

@@ -89,7 +89,13 @@ You can configure the firmware to read a pin on the controller to determine hand
 #define SPLIT_HAND_PIN B7
 ```
 
-This will read the specified pin. If it's high, then the controller assumes it is the left hand, and if it's low, it's assumed to be the right side. 
+This will read the specified pin. By default, if it's high, then the controller assumes it is the left hand, and if it's low, it's assumed to be the right side. 
+
+This behaviour can be flipped by adding this to you `config.h` file:
+
+```c
+#define	SPLIT_HAND_PIN_LOW_IS_LEFT
+```
 
 #### Handedness by Matrix Pin
 

quantum/split_common/split_util.c

@@ -77,7 +77,11 @@ __attribute__((weak)) bool is_keyboard_left(void) {
 #if defined(SPLIT_HAND_PIN)
     // Test pin SPLIT_HAND_PIN for High/Low, if low it's right hand
     setPinInput(SPLIT_HAND_PIN);
+#    ifdef SPLIT_HAND_PIN_LOW_IS_LEFT
+    return !readPin(SPLIT_HAND_PIN);
+#    else
     return readPin(SPLIT_HAND_PIN);
+#    endif
 #elif defined(SPLIT_HAND_MATRIX_GRID)
 #    ifdef SPLIT_HAND_MATRIX_GRID_LOW_IS_RIGHT
     return peek_matrix_intersection(SPLIT_HAND_MATRIX_GRID);