available_color_count is wrong on Windows

[heaths]

Describe the bug
Windows pseudo-TTYs support true color, yes crossterm::style::available_color_count reports 8. You need to query if you're in a pseudo-terminal which, BTW, means you can do just about any VT100 sequence including keyboard support, though I'm not sure what "progressive keyboard support" means.

A better detection of what colors are supported can be found at https://github.com/cli/go-gh/blob/dbd982eba2a041d88d398cce01cb708c4c3503f7/pkg/term/env.go#L70-L72. I worked on that for the GitHub CLI (was refactored into cli/go-gh from cli/cli) along with colleagues on the Windows terminal team that own conhost and conpty - both of which support VT100 sequences since some version of Windows 10 (TH2, maybe? Anything older isn't supported anyway.)

To Reproduce
Steps to reproduce the behavior:

1. Write a simple bin:

   fn main() {
       println!("{}", crossterm::style::available_color_count());
   }

2. Run cargo run

Expected behavior
256 is printed to stdout.

OS

• Windows 11

Terminal/Console

• Windows Terminal; though, conhost.exe supports this - which cmd.exe and powershell.exe/pwsh.exe use by default unless you default to wt.exe, which uses conpty.exe. Effectively, conpty is dead on built-in terminals and shells.

[heaths]

For that matter, just checking TERM is not sufficient on any platform. See helper functions used by the cli/go-ch link above: https://github.com/cli/go-gh/blob/dbd982eba2a041d88d398cce01cb708c4c3503f7/pkg/term/env.go#L170-L184.