A `vendored_stubs` key in the configuration

Which would be a path in the project (usually `stubs`) that are vendored third party stubs, usually generated with stubgen.

These would have a very loose strictness applied to them, as the features from #75 (and other upcoming changes) are tailored for this usage pattern.

Start by locating the configuration handling and strictness logic; the issue does not name specific files or tests. Review the proposed `vendored_stubs` path, typically `stubs`, and the behavior described alongside #75; done means the key is recognized and vendored third-party stubs receive the intended loose strictness.

Written by the indexing model from the issue text.