feat: accept slither.conf.json as alternative default config

[cats2101]

Closes #595

Summary

When no --config-file is provided, Slither will now look for slither.conf.json as a fallback if slither.config.json is not present. If both files exist, slither.config.json takes precedence (preserves current behavior).

This matches the request in the issue: accept slither.conf.json for convenience without breaking anyone.

Changes

• slither/utils/command_line.py: introduce DEFAULT_CONFIG_FILENAMES = ("slither.config.json", "slither.conf.json") and update read_config_file to iterate the tuple.
• slither/__main__.py and slither/tools/slither_format/__main__.py: update --config-file help text. For slither-format, the default changes from the hardcoded string "slither.config.json" to None so the auto-detection chain applies consistently across both entrypoints.
• docs/src/Usage.md: mention both filenames.
• tests/unit/utils/test_command_line.py: new unit tests covering both filenames, precedence, and the no-default case.

Test plan

• New unit tests pass (pytest tests/unit/utils/test_command_line.py -v)
• ruff check passes on modified files
• Behavior check: with only slither.conf.json present, it is picked up and its flags are applied
• Behavior check: with both present, slither.config.json still wins