Allow colored terminal output on Windows if ENABLE_VIRTUAL_TERMINAL_PROCESSING is set

[chausner]

This improves the compatibility of the LoggingFlag::ColoredTerminalOutput setting on Windows.

Windows 10 and later introduced virtual terminal support in conhost and Windows Terminal, see also https://learn.microsoft.com/en-us/windows/console/console-virtual-terminal-sequences. This adds logic to easylogging++ to detect when virtual terminal processing is enabled, to allow colored output if LoggingFlag::ColoredTerminalOutput is set.

Before, easylogging++ would not attempt to output VT colored text even if the user enabled LoggingFlag::ColoredTerminalOutput and the terminal supports it.

With this change, running the all-logs sample in Windows Terminal:

This is a

• Breaking change
• New feature
• Bugfix

I have

• Merged in the latest upstream changes
• Updated CHANGELOG.md
• Updated README.md
• Run the tests

[robloh]

Thanks for this patch! Can confirm it works on Windows 10 with VS2019.

[chausner]

@abumq Any chance of getting this merged? :)