Implement https://forum.nim-lang.org/t/7848#50018 (#17874)

* Implement https://forum.nim-lang.org/t/7848#50018 with just the same
`SuccessX` hint category, build mode on a separate, final line, and
no change to how the mode is spelled for -d:release/-d:danger.

* Change to add a new BuildMode hint category and keep testament in sync
as per comment.

* Add "--hint:buildmode:off" to `defaultHintsOff`.

* Remove as requested.

* As requested for tests clean up.

* Address code review.

* Address code review.

* Mirror db456423116a9b19666f551f4d38aded3964c2e2

compiler/main.nim

@@ -384,9 +384,9 @@ proc mainCommand*(graph: ModuleGraph) =
       when declared(system.getMaxMem): formatSize(getMaxMem()) & " peakmem"
       else: formatSize(getTotalMem()) & " totmem"
     let loc = $conf.linesCompiled
-    let build = if isDefined(conf, "danger"): "Dangerous Release"
-                elif isDefined(conf, "release"): "Release"
-                else: "Debug"
+    let build = if isDefined(conf, "danger"): "Dangerous Release build"
+                elif isDefined(conf, "release"): "Release build"
+                else: "***SLOW, DEBUG BUILD***; -d:release makes code run faster."
     let sec = formatFloat(epochTime() - conf.lastCmdTime, ffDecimal, 3)
     let project = if conf.filenameOption == foAbs: $conf.projectFull else: $conf.projectName
       # xxx honor conf.filenameOption more accurately
@@ -406,10 +406,10 @@ proc mainCommand*(graph: ModuleGraph) =
       "loc", loc,
       "sec", sec,
       "mem", mem,
-      "build", build,
       "project", project,
       "output", output,
       ])
+    rawMessage(conf, hintBuildMode, build)
 
   when PrintRopeCacheStats:
     echo "rope cache stats: "