Co-authored-by: Sergey Vlasov <sigprof@gmail.com>
Co-authored-by: Stefan Kerkmann <karlk90@pm.me>
Co-authored-by: Nick Brassel <nick@tzarc.org>

* Custom matrix lite support for split keyboards

* WIP: matrix -> matrix_common refactor

* Move matrix_post_scan() to matrix_common.c

The prototype of matrix_output_unselect_delay() has been changed as follows.

```c
void matrix_output_unselect_delay(uint8_t line, bool key_pressed);
```

Currently, no keyboard seems to be redefining `matrix_output_unselect_delay()`, so there is no change in the system behavior.

With this change, the keyboard level code can get some optimization hints, for example, the following.

```c
 void matrix_output_unselect_delay(uint8_t line, bool key_pressed) {
     /* If none of the keys are pressed,
      *  there is no need to wait for time for the next line. */
     if (key_pressed) {
 #ifdef MATRIX_IO_DELAY
 #  if MATRIX_IO_DELAY > 0
         wait_us(MATRIX_IO_DELAY);
 #  endif
 #else
         wait_us(30);
 #endif
     }
}
```