* Add executable app for language server integration

Implement a CLI executable app that can be invoked as `jlfmt` or
`julia -m JuliaFormatter`, enabling integration with language servers
(JETLS in particular, which already integrates Runic in a similar way).

The app is registered in Project.toml under `[apps.jlfmt]` to
enable command line invocation when JuliaFormatter is installed as an
executable app (i.e. `pkg> app add JuliaFormatter`).

The implementation follows Runic's executable app interface design:
- Similar command-line options (`-h/--help`, `--check`, `--diff`, `--inplace`, etc.)
- Recursive directory scanning for Julia files
- Support for stdin/stdout piping
- Progress reporting with colored output
- Git diff integration for showing formatting changes
- Multi-threaded formatting for multiple files

Notable differences from Runic:
- No `--lines` option for partial file formatting (not yet implemented)
- Extensive formatting options exposed via CLI flags (style, indent, margin, etc.)
- Integration with .JuliaFormatter.toml configuration files
- `--config-priority` flag for language server integration

Comprehensive CLI documentation is also added (docs/src/cli.md).

* update docs to explain `--config-dir` option