Changes:
- Add dedicated exception for call_ref invoking a WasmJSFunction.
- Small restructuring of read_value_type.
- Change HeapType::kLastSentinel to point to the last valid type,
  update is_valid().
- Remove redundant DCHECK from ValueType constructors.
- Rename a few section-related macros in module-decoder-unittest.cc,
  add a small test.
- Rename "Simd128" -> "s128" in error message.
- Write some documentation, mostly in value-type.h and wasm-subtyping.h.

Bug: v8:7748
Change-Id: I4fc4826fbdeac50e21ef524787c2024d7aa1b3b2
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2424139
Commit-Queue: Manos Koukoutos <manoskouk@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70118}