Add .gitattributes file to override GitHub's language detection from Makefile to Rust

[Copilot]

GitHub was incorrectly identifying the Shimmy repository as a "Makefile" project instead of "Rust" due to the overwhelming number of build-generated dependency files. The Rust build system creates 530+ .d files with Makefile syntax that were dominating GitHub's language detection algorithm, despite the repository containing 57 actual Rust source files.

This PR adds a .gitattributes file that:

• Prioritizes Rust detection by explicitly marking *.rs files as linguist-detectable=true
• Excludes build artifacts by setting *.d dependency files and Makefile variants as linguist-detectable=false
• Marks generated files like target/, Cargo.lock, and profiling data as linguist-generated=true
• Preserves important config files by keeping .toml, .yml, and .yaml files detectable

The configuration has been tested with git check-attr to ensure proper pattern matching, and the project continues to build successfully. This follows Git linguist best practices for language override and addresses the core issue with minimal changes.

Fixes #55.

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