Truecolor terminal capability detection for Windows (#53257)
```
for i in 0:255
print("\x1b[38;2;$i;0;0m█")
end
```
shows on Windows Terminal
so it does have true-color support.
With this PR, `Base.ttyhastruecolor()` returns `true` on Windows
Terminal and Cygwin64 Terminal, those are the ones I tried out.
Notes:
https://devblogs.microsoft.com/commandline/24-bit-color-in-the-windows-console/
says
> in Windows 10 Insiders Build # 14931, we’ve updated the Windows
Console to support full, glorious 24-bit RGB true color!
