Fix check compat should early exit

[kunitoki]

This pull request introduces a new minor version (0.21.0) and focuses on improving the handling of breaking changes in the CLI, ensuring output files are not written when breaking changes are detected during compatibility checks. It also updates the test suite to validate this behavior and makes minor documentation updates.

CLI breaking change handling improvements

• Updated the CLI (src/tsujikiri/cli.py) so that if breaking changes are detected with --check-compat, the process exits with status 1 before generating any output files, ensuring no output is written in this scenario. [1] [2]

Test suite enhancements

• Added a new test (test_breaking_change_does_not_write_output_file) to verify that output files are not written when breaking changes are detected, and refactored some existing tests for clarity and coverage. [1] [2]

Version update

• Bumped the package version to 0.21.0 in src/tsujikiri/__init__.py.

Documentation updates

• Updated badge URLs and coverage tree links in README.md to use a new query parameter. [1] [2]