Add sqlite3_web_vfs_list() to JS API. Corrected OPFS VFS's registering itself as...

Add sqlite3_web_vfs_list() to JS API. Corrected OPFS VFS's registering itself as the default VFS. speedtest1-worker now uses the xDelete() of both the default VFS and OPFS, to avoid that it starts up with a persistent OPFS test db (the native app calls unlink(), but that unlink call operates on a different virtual filesystem than the OPFS VFS).

FossilOrigin-Name: 2ec7e09139a510b9fd29e4c97283b20740a00f369193c6fecbb734f187e81b48