merge BuildMode into SuccessX, remove code duplication w drnim, add useful info to successx, add gc to compilesettings (#18252)

* merge BuildMode into SuccessX, add more info

* refactor duplicated with drnim

* fixup

* address comment

lib/std/compilesettings.nim

@@ -32,6 +32,7 @@ type
     backend           ## the backend (eg: c|cpp|objc|js); both `nim doc --backend:js`
                       ## and `nim js` would imply backend=js
     libPath           ## the absolute path to the stdlib library, i.e. nim's `--lib`, since 1.5.1
+    gc                ## gc selected
 
   MultipleValueSetting* {.pure.} = enum ## \
                       ## settings resulting in a seq of string values