Use explicit ordinals for static unions in FIDL

As part of FTP-048, unions will be migrated to use explicit
ordinals. This updates all FIDL unions to use the explicit ordinal
syntax.

Bug: fuchsia:40859
Change-Id: Ie4cab2101cd7340affe8148cad7989a86776bbcd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1899910Reviewed-by: Wez <wez@chromium.org>
Commit-Queue: Felix Zhu <fcz@google.com>
Cr-Commit-Position: refs/heads/master@{#714009}

build/fuchsia/fidlgen_js/test/simple.fidl

@@ -49,9 +49,9 @@ struct LargerStructWithArray {
 };
 
 union UnionOfStructs {
-  StructWithBool swb;
-  StructWithUint swu;
-  LargerStructWithArray lswa;
+  1: StructWithBool swb;
+  2: StructWithUint swu;
+  3: LargerStructWithArray lswa;
 };
 
 struct StructOfMultipleUnions {