process: make exitCode configurable again

This change was done in nodejs#44711, and it's not clear it was intentional.
It caused nodejs#45683, and also makes it impossible to mock out the exitCode
in tests.

Filing this PR per nodejs#44711 (comment)

Fixes nodejs#45683.

lib/internal/bootstrap/node.js

@@ -129,7 +129,7 @@ process.domain = null;
       exitCode = code;
     },
     enumerable: true,
-    configurable: false,
+    configurable: true,
   });
 }
 process._exiting = false;