Type checking with expected type errors

[soutaro]

This PR is to add --with-expectations and --save-expectations option to steep check.

• When --with-expectations option is specified, it confirms that if reported type errors are expected ones.
• When --save-expectations option is specified, it saves current type errors as expectations file.

The options are for running type checking in CI for the cases:

1. When type errors are expected (this would happen when you are writing RBS files)
2. When type errors cannot be removed (this would happen because of the poor experience provided by Steep 😢)

The workflow we assume is:

1. Run Steep and confirm if the current type errors are what you want.
2. Save the current status with --save-expectations.
3. Run Steep with --with-expectations in CI.

ToDos

• What if CLI arguments are given to steep check???
• Update test:output tools.

[palkan]

Hey @soutaro,

Just found this feature—really helpful!

What about making it possible to specify expectations_path in the Steepfile to always use it (without explicitly passing --with-expectations)?

Extracted into an issue #468