Add support for SetOutputMode other than OutputNormal for Windows Terminal

[AlejandroSuero]

I am a bit new to golang so I am not sure how will it be possible to detect if the user is using Windows terminal instead of the cmd.

Windows terminal does support RGB, when using WSL I can use the colours from RGBToAttribute, but when I am using PowerShell it detects that I am using Windows so it outputs normal mode instead.

In WSL:

In PowerShell:

I want to achieve the same behaviour to get more control over colours.

This is the code I wrote for this behaviour:

// ... code

// setting tabs as '·' rune and using the a dark gray as foreground
termbox.SetCell(col, row, rune('·'), termbox.RGBToAttribute(62, 64, 66), termbox.ColorDefault)

// ...

// setting the output mode to RGB in the main loop
termbox.SetOutputMode(termbox.OutputRGB)

//...