A new constructor has been added which accepts a enum type BG_SPI_SLOT which can be either:
ST7789::BG_SPI_FRONT or ST7789::BG_SPI_BACK.
This selects the correct CS/BL pins for the respective breakout-garden SPI slot.
Additionally the PWM'd backlight is turned *on* by default to avoid any confusion when the user does not set a value.