Add type checking configuration to dsl

[alts]

From the commit message:

This allows the project owner to specify the strictness of type
checks in their project. The naming of the new options draws upon
the structure of similar options in mypy[0] and the naming of the
typing errors themselves.

These two errors are very common ones that arise when trying to add
steep to an existing untyped project.

[0] https://mypy.readthedocs.io/en/stable/config_file.html

I tried adding steep to a few projects just to see what the process of adding types to an existing project was. These two typing errors came up regularly, and could be an impediment to adding typing gradually to an existing project.

[soutaro]

@alts Thanks!

• Could you add a unit test for the new DSL syntax? I think adding a test case in project_test.rb makes sense.
• Could you check the test failures?

[soutaro]

I took a look at the smoke test failures. The errors are because of that you implemented the method error filtering, which hasn't been implemented for months... 🙇‍♂️

I'd like to suggest to implement two major modes for type checking options, strict and moderate (or you may find better names.) In the smoke tests, we want to use the strict mode, like:

target ... do
  ...

  typing_options :strict
end

[alts]

In that model, would moderate be the default settings?

[alts]

The latest change introduces three strictness levels. strict, default (what existed before), and lenient. I can make moderate an alias for default, if you'd like. I also wonder about the long-term usefulness of lenient as a category rather than being accomplished through individual toggles.

I some assertions to steepfile_test.rb. I wasn't sure how to go about adding a useful test to project_test.rb. I also realize now that having the filtering of errors done in Drivers::Check probably isn't the best home. I tried moving it to Project::Target, which seems better for testability. I wasn't sure about how equivalent TypeCheckStatus#type_check_sources and Target#source_files were. It seems like there's some filtering occurring.

[soutaro]

Thanks!

:lenient sounds good! And it seems that adding assertions on steepfile_test makes sense.

I agree that having the filtering in Drivers::Check doesn't look the best, but I'm also not very sure where is the best place... So, I think we can continue with your implementation.