feat(kscan): Kconfig for optional scan delay.

Add optional Kconfig setting to delay scanning after each
output column is set, and inputs are read, to allow inputs
to "settle" after the last column is set back to inactive.

app/drivers/kscan/kscan_gpio_matrix.c

@@ -250,6 +250,10 @@ static int kscan_matrix_read(const struct device *dev) {
             LOG_ERR("Failed to set output %i inactive: %i", o, err);
             return err;
         }
+
+#if CONFIG_ZMK_KSCAN_MATRIX_WAIT_BETWEEN_OUTPUTS > 0
+        k_busy_wait(CONFIG_ZMK_KSCAN_MATRIX_WAIT_BETWEEN_OUTPUTS);
+#endif
     }
 
     // Process the new state.