Add missing FileSystemWritableFileStream API (#7745)
Add missing FileSystemWritableFileStream API (#7745)

This PR is a part of a project to add missing interfaces and interface features to BCD that are from a spec in Editor's Draft or more and is supported in at least one browser.  This particular PR adds the missing FileSystemWritableFileStream API, populating the results using data from the [mdn-bcd-collector](https://mdn-bcd-collector.appspot.com) project (v1.1.6).

Spec: https://wicg.github.io/file-system-access/#api-filesystemwritablefilestream

IDL: https://github.com/w3c/webref/blob/master/ed/idl/file-system-access.idl

Tests Used: https://mdn-bcd-collector.appspot.com/tests/api/FileSystemWritableFileStream
mdn/browser-compat-data,mdn/browser-compat-data,chrisdavidmills/browser-compat-data,chrisdavidmills/browser-compat-data