js-runtime: Use the same tag for both func_js and func_zig

There is no technical different between the ways they are used. So its
better to merge them.

This commit further removes the internal ValueTag which worked under
Tag. On doing that tag type ``.ref`` has been renamed to ``.object``.

js-runtime/src/js-runtime.js

@@ -173,7 +173,6 @@ const zig = {
     switch (block.getU8(0)) {
       case 0:
       case 6:
-      case 7:
         return values[block.getU64(8)];
         break;
       case 1: