wasm: when building in -O0 mode (typical dev mode), use -sASSERTIONS=2, else -sASSERT...

wasm: when building in -O0 mode (typical dev mode), use -sASSERTIONS=2, else -sASSERTIONS=0, in response [https://github.com/emscripten-core/emscripten/pull/23629/commits/7e3e35cbff9c5688eacb3cddc5045f872d744efd|Emscripten checkin 7e3e35cbff9], which adds assertions to check for the condition reported in [https://github.com/emscripten-core/emscripten/issues/23420|Emscripten ticket 23420]. Update some unrelated JS-side internal docs.

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