[wasm2c] Add the very beginning of a wasm2c implementation and test harness (#8763)

This change adds an extremely bare-bones wasm2c implementation and test
harness.

Most of the code here is imported directly from WABT, particularly the
boilerplate code and tests. The only substantive new code is in
src/tools/wasm2c/* and scripts/test/wasm2c.py.

I tried to prune this down to be as minimal as possible while still
providing a useful checkpoint, but I'm not sure I succeeded. Please feel
free to offer suggestions if you see more ways to shrink this down.