The bl module has a file called `BufferList.js` that is the buffer list
without node streams, the idea being you can use it in the browser without
having to pull in a readable streams implementation, which has become more
necessary since webpack 5 stopped bundling shims for node internals.

The main `bl.js` exports a class that extends both the buffer list and
the node Duplex stream.  It also has named exports for `BufferListStream`
(the same as the default export) and `BufferList` (the same as the class
from `BufferList.js`).

I've attempted to represent them here.  TS doesn't have multiple inheritance
so I'm not sure how to represent the `BufferListStream` class without
duplicating the methods of `BufferList`.