Replace createWriter, FileSystemWriter with createWritable, WritableFileStream

[oyiptong]

This adds specs for WritableFileStream and the factory and options.
It links to WritableStreams related specs, with an inheritance
relationship.

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[oyiptong]

@domenic, what do you think of this PR as a response to #19?

In particular, what do you think of the initialization of position in createWritable(). This would be used instead of a seek() method on the WritableFileStream.

[mkruisselbrink]

I think this is a good start, although I do wonder about the exact API shape in a number of places.... Interested to hear what domenic's thoughts are.

[domenic]

You shouldn't define write() and close(); users should call those via the writer.

To customize the behavior of the writer, instead you need to define (in the definition of createWritable) what the underlying sink algorithms are. See https://fetch.spec.whatwg.org/#concept-construct-readablestream and its call sites for some rough guidance (although that's for readable; for writable you'd be using https://streams.spec.whatwg.org/#create-writable-stream).

[oyiptong]

What do you think of defining write() and truncate(), so that FileSystemWritableFileStream can be used directly?

For instance, one could do:

const stream = await handle.createWritable();
await stream.write(new Blob(['foo']); // New method, peer to `seek` and `truncate`.
await stream.close(); // Overrided method from WritableStream.

At the same time, the underlying sink algorithms will be implemented to accept WriteParams to execute commands, or to take byte-like data.

[oyiptong]

Closes #19