Migrate CLI argument parsing from builder to derive API

[Wilfred]

This PR refactors the command-line argument parsing in difftastic to use clap's derive API instead of the builder API, improving code maintainability and consistency.

Summary

The argument parsing logic has been migrated from clap's builder pattern (using Command, Arg, and ArgAction) to the derive API (using the Parser derive macro). This modernizes the codebase and makes argument definitions more declarative and easier to maintain.

Key Changes

• Migrated to clap derive API: Replaced manual app() function that built arguments with a #[derive(Parser)] struct Args with field-level attribute macros
• Improved error handling: Created dedicated error reporting functions (arg_error, bad_arguments) that use clap's error formatting for consistency
• Refactored argument parsing logic:
  • Extracted parse_language_override() and parse_glob() as value parsers
  • Created parse_numbered_env_vars() to handle numbered environment variables (DFT_OVERRIDE_1 through DFT_OVERRIDE_9)
  • Extracted combine_overrides() to group adjacent overrides by language
• Added new types:
  • OnOff enum with ValueEnum derive for on/off style arguments
  • LanguageOverrideArg type alias for parsed override values
  • PathArgs enum to represent different calling conventions
• Extracted path parsing: Created parse_paths() function to interpret positional arguments according to supported calling conventions
• Enhanced validation: Improved error messages for invalid arguments, including better reporting of which environment variable caused an error
• Updated dependencies: Added derive feature to clap in Cargo.toml
• Added ValueEnum derives: Applied to ColorOutput, DisplayMode, and BackgroundColor for clap integration

Notable Implementation Details

• The Args struct uses #[command(...)] attributes to configure the command itself (name, version, help text, etc.)
• Field-level doc comments serve as help text with verbatim_doc_comment to preserve formatting
• The after_help() function was extracted to provide the examples shown at the end of --help
• Error handling now uses clap's ErrorKind enum for proper error categorization
• The refactoring maintains backward compatibility with all existing calling conventions and environment variables
• Added comprehensive test coverage for new error cases (invalid overrides, wrong number of arguments, etc.)

https://claude.ai/code/session_0162QcLTys6tv1s1Lg1jw2i5

[joyously]

Do the tests cover absolute versus relative paths, different separators for Windows, and UTF8 characters in filenames?

[Wilfred]

@joyously I was exploring using derive because it's more concise and allows me to use doc comments for argument descriptions, but I don't think it's worth it after playing with this code. The explicit API is much easier to reason about for a CLI with multiple usage conventions.

It has found some interesting argument bugs though.

Regarding your concerns about funky paths, have you seen any issues with current difftastic?

[joyously]

No I have not used difftastic, because it's so large, and I have not been coding much... But I have been following the Jujutsu project which uses `clap`, and they have had all the issues I mentioned.