Use BEP to get information about builds in progress

(From #17)

We should leverage Bazel's [Build Event Protocol](https://docs.bazel.build/versions/master/build-event-protocol.html) in the extension so that we can provide various UI feedback about builds the user launches, such as (just some random ideas):

* Progress indicators
* Scraping common error/warning patterns from command output and surfacing the information in the Problems view
* showing the full Bazel output in a nicely formatted webview

Like the debugger, we can "vendor" the [protos](https://github.com/bazelbuild/bazel/blob/master/src/main/java/com/google/devtools/build/lib/buildeventstream/proto/build_event_stream.proto) we need and generate the TS interfaces for them.

Start by reading the linked Bazel Build Event Protocol documentation and the build_event_stream.proto mentioned in the issue, then inspect the extension entry points for launched Bazel builds. The issue does not name files or tests and lists several possible UI features, so the intended scope and definition of done need to be clarified before implementation.

Written by the indexing model from the issue text.