Expose sqlite3_vfs_unregister() to WASM and unregister kvvfs in Worker threads to...

Expose sqlite3_vfs_unregister() to WASM and unregister kvvfs in Worker threads to avoid its unintended use there (in contexts other than local/sessionStorage). Correct registration of window functions, extend oo1.DB.createFunction() to support window functions, and add window function tests to tester1.js. Correct an incorrect 1-arg handling case for DB.exec(). Add per-test assertion counts to tester1.js.

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