Add guide: Conditional compilation

TODO

Tauri exposes several environment variables to the beforeDevCommand and beforeBuildCommand, allowing frontend bundlers to perform conditional compilation. You can use this to write platform-specific JS code.

Details

JavaScript

• only available with some bundlers (but most) -> called treeshaking

• the cli exposes a set of environment variables

  • TAURI_PLATFORM - same as target_os
  • TAURI_ARCH - same as target_arch
  • TAURI_FAMILY - same as target_family
  • TAURI_PLATFORM_VERSION - version of the OS
  • TAURI_PLATFORM_TYPE - "Linux" | "Darwin" | "Window_NT" same as nodejs os.type()
  • TAURI_DEBUG - same as debug_assertions

• quick example

Rust

• link to conditional compilation guide https://doc.rust-lang.org/reference/conditional-compilation.html
• quick intro cfg! and #[cfg()]
• quick intro features
• target a specific OS (target_os and target_family)
• target a specific CPU architecture
• cfg(debug_assertions)

Locate the documentation section for build and development commands, then review the linked Rust conditional-compilation guide. Add a guide covering the listed JavaScript environment variables, treeshaking, and Rust cfg features, targets, architectures, and debug assertions, with quick examples for both languages.