constants: freeze the constants object

Constants ought to be constant. The primary goal of this commit is to
make constants exposed in require('constants') immutable, as they were
prior to [email protected], and as the constants exposed on fs.constants,
crypto.constants, etc. are.

Since this is implemented by using Object.freeze, it also has the side
effect of making the entire exports of require('constants') immutable,
so no new constants can be defined on the object in userland.

PR-URL: #19813
Reviewed-By: James M Snell <[email protected]>
Reviewed-By: Tiancheng "Timothy" Gu <[email protected]>
Reviewed-By: Rich Trott <[email protected]>
Reviewed-By: Gus Caplan <[email protected]>
Reviewed-By: Ruben Bridgewater <[email protected]>
Reviewed-By: Ali Ijaz Sheikh <[email protected]>
Reviewed-By: Matteo Collina <[email protected]>

Author: bengl

Diff for: lib/constants.js

@@ -32,3 +32,4 @@ Object.assign(exports,
               constants.os.signals,
               constants.fs,
               constants.crypto);
+Object.freeze(exports);

Diff for: test/parallel/test-constants.js

@@ -24,3 +24,5 @@ assert.ok(binding.crypto);
     }
   });
 });
+
+assert.ok(Object.isFrozen(constants));