Support baselines, i.e., ignore existing errors for incremental adoption

Baselines would allow developers to introduce Ruff to their codebase incrementally by ignoring existing errors and address them as they reappear when making changes to the code.

Supported by some tools in the flake8 ecosystem:

• Bandit: https://bandit.readthedocs.io/en/latest/start.html#baseline
• Flakeheaven: https://wemake-python-styleguide.readthedocs.io/en/latest/pages/usage/integrations/flakeheaven.html#legacy-first-1

A very basic proposal for this in ruff:

In the CLI

ruff . --exit-zero --format baseline > .ruff_baseline
ruff . --baseline .ruff_baseline  # doesn't report any errors

In pyproject.toml

[tool.ruff]
baseline = ".ruff_baseline"