Handle `exit` in main REPL instead of ExitBuiltin.Run()

app/builtin/builtin.go

 }
 
 func (self *ExitBuiltin) Run(ctx *core.Context) core.ActionResult {
-	os.Exit(0)
-	return wrapEs(0)
+	return core.ActionResult{
+		Code:       core.ActionResultCodeExit,
+		ExitStatus: 0,
+	}
 }
 
 //

app/core/core.go

 	ActionResultCodeNoop ActionResultCode = iota
 	ActionResultCodeBuiltinCommand
 	ActionResultCodeExternalCommand
+	ActionResultCodeExit
 )

app/main.go

 
 		result := runnable.Run(ctx)
 		switch result.Code {
+		case core.ActionResultCodeExit:
+			os.Exit(int(result.ExitStatus))
 		case core.ActionResultCodeNoop:
 		case core.ActionResultCodeBuiltinCommand:
 		case core.ActionResultCodeExternalCommand: