Add support for swap hands

[0xcharly]

Example for 4x6

#ifdef SWAP_HANDS_ENABLE
__attribute__((weak))
const keypos_t PROGMEM hand_swap_config[MATRIX_ROWS][MATRIX_COLS] = {
    /* Left hand, matrix positions */
    {{0, 5}, {1, 5}, {2, 5}, {3, 5}, {4, 5}, {5, 5}},
    {{0, 6}, {1, 6}, {2, 6}, {3, 6}, {4, 6}, {5, 6}},
    {{0, 7}, {1, 7}, {2, 7}, {3, 7}, {4, 7}, {5, 7}},
    {{0, 8}, {1, 8}, {2, 8}, {3, 8}, {4, 8}, {5, 8}},
    {{2, 9}, {4, 9}, {5, 9}, {1, 9}, {0, 9}, {3, 9}},
    /* Right hand, matrix positions */
    {{0, 0}, {1, 0}, {2, 0}, {3, 0}, {4, 0}, {5, 0}},
    {{0, 1}, {1, 1}, {2, 1}, {3, 1}, {4, 1}, {5, 1}},
    {{0, 2}, {1, 2}, {2, 2}, {3, 2}, {4, 2}, {5, 2}},
    {{0, 3}, {1, 3}, {2, 3}, {3, 3}, {4, 3}, {5, 3}},
    {{0, 4}, {3, 4}, {2, 4}, {5, 4}, {4, 4}, {2, 4}},
    };
#endif

[0xcharly]

Closing for now. There's no real point in increasing the complexity of the firmware for a feature no one has ever requested.