Fixes #21376

This makes output = json in mypy.ini take effect by registering output as a string-valued config option. Options.output defaults to None, so without an explicit config parser type entry it was treated as unrecognized and ignored.

This is intentionally the narrow version of the fix: it does not change the general handling of all None-default options.

I also added a config parser unit test that parses a temporary mypy.ini with output = json and verifies Options.output is set without stderr output.

Validation:

• .venv/bin/python -m pytest mypy/test/test_config_parser.py
• .venv/bin/python -m black --check mypy/config_parser.py mypy/test/test_config_parser.py
• .venv/bin/python -m ruff check mypy/config_parser.py mypy/test/test_config_parser.py
• git diff --check

Note: this replaces #21416, which I could not reopen after cleaning up the branch history.