feat: add support for pre-compiled binaries

[vihu]

Summary

Implements pre-compiled binary distribution allowing users to install and use the plugin without requiring a Rust toolchain. The CI/CD pipeline builds release binaries for Linux and macOS (both x86_64 and aarch64), and a new Lua build module handles automatic download with graceful fallback to source compilation.

Changes

• Add GitHub Actions release workflow that builds binaries for 4 platforms (linux-x86_64, linux-aarch64, macos-x86_64, macos-aarch64) on tag push
• Create lua/penview/build.lua module with platform detection, binary download/extraction, version tracking, and source build fallback
• Update lua/penview/init.lua to check bin/penview before target/release/penview
• Update lua/penview/websocket/init.lua to check bin/lua/websocket_ffi.so before source-built location
• Add /bin to .gitignore for downloaded binaries
• Rewrite README.md installation section with examples for lazy.nvim, packer.nvim, vim-plug, and source builds