v2.0.0

Requires Dart sdk: '>=2.18.0'

• New package:lint/strict.yaml
• New package:lint/casual.yaml
• New package:lint/package.yaml
• Deprecate package:lint/analysis_options.yaml (uses strict.yaml)
• Deprecate package:lint/analysis_options_package.yaml (uses package.yaml)
• Disable use_decorated_box
• Enable unnecessary_null_aware_operator_on_extension_on_nullable
• Enable unnecessary_to_list_in_spreads