This is a first small step for implementing the memory64 proposal:
1. Add a feature flag.
2. Add the 0x04 and 0x05 limits flag for memory64.
3. Read memory limits as LEB-encoded u64 (instead of u32) if a memory64
   limit flag was read.
4. Unify {MaximumFlag} and {MemoryFlag}, which was used inconsistently
   before.
5. Add test for memory limits encoded with >5 bytes.
6. Move some macros from module-decoder-unittest.cc to wasm-macro-gen.h.

Note that still the same limits for the maximum number of pages applies
as before, i.e. you cannot specify a memory >4GB yet. But you can encode
that small number in >5 bytes.

R=manoskouk@chromium.org

Bug: v8:10949
Change-Id: I90a4f08426ae714a67440281785eb00cfc24a349
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2423712
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Manos Koukoutos <manoskouk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70110}