Allow filtering extensions by publisher

[jthomasmock]

It would be nice to be able to filter the extension list by publisher.

Per this resource, VS Code Marketplace offers this:

1. In IDE:

2. On Web: https://marketplace.visualstudio.com/search?term=publisher%3Amicrosoft&target=VSCode&category=All%20categories&sortBy=Relevance

[netomi]

works great!

I wonder if we should not add documentation somewhere about the possible search patterns.