I wrote up how to compile SQLite to WASM and also include an extension: https://github.com/llimllib/wasm_sqlite_with_stats

You can play with the wasm-compiled extension at the version of the sandbox I compiled here: https://llimllib.github.io/wasm_sqlite_with_stats/

I tried to be thorough with the writeup, so hopefully it helps somebody if that's something they need.