This commits a bazel-based build system for C#
using `rules_dotnet`. External dependencies are managed
via `paket`, and updates to the generated bazel files
are done via `./update-deps.sh`.
We're providing our own (minimal) test runner for `xunit`
tests.
This means that `dotnet paket update` will pull in the
latest matching versions of all dependencies, except
the pinned roslyn versions.
GitInfo also remains pinned, as we're removing that
dependency when converting the build to bazel anyways.
This removes a whole bunch of transitive dependencies
we no longer need, resulting in a smaller distribution,
and hopefully a smaller build.
