sorbet: RSpec uses dynamic `be_` and `have_` matchers

- Some RSpec `be_` and `have_` matchers are dynamically generated[1],
  meaning that `be_empty` etc. are unknown to Sorbet. So, add a Tapioca
  compiler to generate RBIs for these matchers that are used in our
  tests but not elsewhere defined.
- This generates the same kind of RBI file as `Homebrew::Service`
  methods: all `T.untyped`.

[1]: https://github.com/rspec/rspec/blob/8098cb121a440cbc1f9dc4581ac65edbd9f001c8/rspec-expectations/lib/rspec/matchers.rb#L25-L59