Add support for publishing VS Code extensions

[winstliu]

Just curious if there's any interest in supporting this, to round out the ADO+VS+VSC trio.

[winstliu]

@jessehouwing wondering if you have any opinions here. Trying to use vsce natively is causing a bit of a headache: microsoft/vscode-vsce#976.

[jessehouwing]

I wouldn't mind having it added, but I have very little experience with this. It would likely push the extension past the max file size again. Might be better to fork an extension that adds this functionality.

[winstliu]

While I don't really have the time to work on this, I'll note that the max file size was an arbitrary limitation that I fixed upstream: microsoft/tfs-cli#458

So that should no longer be an issue :)

[jakebailey]

I'm also wanting this, though I'm not sure what you'd do about vsce other than to download it from npm on the fly? I it has native components and lots of flags.

In a way I'd rather have a task whose sole job it is to provide the PAT from the service connection for use in later steps, that way I can just make use of the PAT myself.