Sleep mode for the st7789v

**Describe the problem you have/What new integration you would like**

Please add public method sleep(bool onoff) for the st7789v module.

**Please describe your use case for this integration and alternatives you've tried:**

Something like that:
void sleep(bool onoff) {
this->write_command_(onoff ? ST7789_SLPIN : ST7789_SLPOUT);
//etc
}
It's useful for displays without backligth pin.

**Additional context**

Start by locating the st7789v module and reading its existing public display-control methods and command-writing path. Add the requested sleep(bool onoff) behavior using the ST7789_SLPIN and ST7789_SLPOUT commands, then verify that displays without a backlight pin can enter and leave sleep mode.

Written by the indexing model from the issue text.