support extension categories/tags and searching by them

Created by: sqs

Extension manifests can now have categories and tags properties (both string[]). See updated docs in this PR.

The extension registry search now supports search queries containing category:"C" and tag:"T", such as category:"Programming languages". This query matches extensions that list Programming languages as a category in their extension manifest. Any additional query terms will be matched against the title and extension ID (as they normally would be).

fix #1494