improve IsColorTerminal windows support

[hermannm]

Changes

• add separate IsColorTerminal implementation for Windows
  • checks for the ENABLE_VIRTUAL_TERMINAL_PROCESSING flag, to see if ANSI colors are supported (https://learn.microsoft.com/en-us/windows/console/console-virtual-terminal-sequences)
  • also checks for ENABLE_PROCESSED_OUTPUT, since that should be set when ENABLE_VIRTUAL_TERMINAL_PROCESSING is set (https://learn.microsoft.com/en-us/windows/console/setconsolemode#parameters)
  • similar implementation as libuv (used by Node.js)
• add checks for the NO_COLOR and FORCE_COLOR environment variables
• remove IsCygwinTerminal check
  • this is redundant, since windows.GetConsoleMode already fails for such terminals, so the behavior is the same (tested this on my Windows machine)
  • this lets us drop the dependency on go-isatty in favor of golang.org/x/term - I would say that golang.org dependencies are preferable
• fix minor typos and formatting in go.mod and jsoncolor.go

[neilotoole]

LGTM.

[neilotoole]

@hermannm Thanks once again for the excellent PR. We could cut a new release now, or do you have any other changes in mind coming soon?

[hermannm]

@neilotoole I don't have anything more in mind for now, so if you'd like to push a v0.7.1 tag for example, that'd be great!

[neilotoole]

Tagged and released 👍