Get fiddle db export working for OPFS VFS. Add root dir handle to the main OPFS VFS...

Get fiddle db export working for OPFS VFS. Add root dir handle to the main OPFS VFS worker to enable creation of certain utility functions without delegating to the async worker. Add sqlite3.capi.sqlite3_wasm_rc_str() to map integer result codes back to their SQLITE_xxx counterparts. Minor doc touchups.

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