nimsuggest: add a command that returns the project file (#12411)

This can be used to let nimsuggest users know which project is
nimsuggest being in charge of, so they can know when should a new
nimsuggest instance be spawned.

An additional feature is to debug the project file finding feature.

compiler/options.nim

@@ -118,7 +118,7 @@ type
 
   IdeCmd* = enum
     ideNone, ideSug, ideCon, ideDef, ideUse, ideDus, ideChk, ideMod,
-    ideHighlight, ideOutline, ideKnown, ideMsg
+    ideHighlight, ideOutline, ideKnown, ideMsg, ideProject
 
   Feature* = enum  ## experimental features; DO NOT RENAME THESE!
     implicitDeref,
@@ -709,6 +709,7 @@ proc parseIdeCmd*(s: string): IdeCmd =
   of "outline": ideOutline
   of "known": ideKnown
   of "msg": ideMsg
+  of "project": ideProject
   else: ideNone
 
 proc `$`*(c: IdeCmd): string =
@@ -725,6 +726,7 @@ proc `$`*(c: IdeCmd): string =
   of ideOutline: "outline"
   of ideKnown: "known"
   of ideMsg: "msg"
+  of ideProject: "project"
 
 proc floatInt64Align*(conf: ConfigRef): int16 =
   ## Returns either 4 or 8 depending on reasons.