[wasm][memory64] Fix typing of memory.size

memory.size returns in i64 if memory64 is enabled.
This CL fixes typing and adds a decoder test. Execution will be tested
and fixed in a follow-up CL.

R=manoskouk@chromium.org

Bug: v8:10949
Change-Id: I15818a6273b579d0faacec7f77dc813ae9ba218f
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2632593


Reviewed-by: Manos Koukoutos <manoskouk@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#72132}