An initial attempt to plugging opfs-wl into the build but its initial handshake with...

An initial attempt to plugging opfs-wl into the build but its initial handshake with the async half collides with the opfs VFS's handshake, causing bootstrapping to fail miserably. We'll need to either devise a handshake which can differentiate between the two instances or we'll need to preprocess sqlite3-opfs-async-proxy into two copies. Move the (now) three copies of some common code shared by the opfs pieces into a preprocessor #include.

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