dartSupportsAnsiColor top-level property Null safety

Read only. Chalk does not use it's value although in some situation a user might use this to set the Chalk ansi color level (or something) I have left it here only for completeness. Within the common debug consoles this variable is essentially worthless because they return FALSE from both VSCode debug console and IntelliJ/Android Studio debug consoles. (This is because dart just checks for 'xterm' string being present in the 'TERM' environmental variable of the console..) See https://github.com/dart-lang/sdk/issues/31606 and https://github.com/dart-lang/sdk/issues/41770 for more info.