[Grbl_Esp32 PR#89] Add basic platformio config.

Add basic platformio config file with corresponding gitignore file.

Assuming, device is at /dev/ttyUSB4 to compile code and flash it call
> $ pio run –target=upload –upload-port=/dev/ttyUSB4

and to upload SPIFFS command would be

> $ pio run –target=uploadfs –upload-port=/dev/ttyUSB4

Note: I used for testing a ESP32-T board – using baudrate 512000 was working for SPIFFS stable enough, but for code it sometimes failed. If you have same issue, just reduce baudrate in platformio.ini e.g. to 115200.