Refactor colour handling to prepare for per-reporter colour modes

This includes always compiling the ANSI and None colour
implementations, as they don't need to touch any platform
specific APIs, and removing their respective compile-time
configuration options.

Because the Win32 colour implementation requires Win32-specific
APIs, it is still hidden behind a compile-time toggle,
`CATCH_CONFIG_COLOUR_WIN32` (renamed from `..._COLOUR_WINDOWS`).

The commandline options for colours were also changed. The
option now uses different name, and allows to select between
different implementations, rather than changing whether
the compiled-in colour implementation is used through
"yes/no/default" options.

docs/release-notes.md

@@ -179,6 +179,13 @@ v3 releases.
   * To support this, the `-r`, `--reporter` flag now also accepts optional output destination
   * For full overview of the semantics of using multiple reporters, look into the reporter documentation
   * To enable the new syntax, reporter names can no longer contain `::`.
+* Console colour support has been rewritten and significantly improved
+  * The colour implementation based on ANSI colour codes is always available
+  * Colour implementations respect their associated stream
+    * previously e.g. Win32 impl would change console colour even if Catch2 was writing to a file
+  * The colour API is resilient against changing evaluation order of expressions
+  * The associated CLI flag and compile-time configuration options have changed
+    * For details see the docs for command-line and compile-time Catch2 configuration
 
 
 ### Fixes