feat: add JavaScript backend for browser/Node.js compilation

Implement Phase 1 of the browser/frontend plan with a complete JS
code generator that compiles Lux programs to JavaScript:

- Basic types: Int, Float, Bool, String, Unit → JS primitives
- Functions and closures → native JS functions with closure capture
- Pattern matching → if/else chains with tag checks
- ADT definitions → tagged object constructors
- Built-in types (Option, Result) → Lux.Some/None/Ok/Err helpers
- Effects → handler objects passed as parameters
- List operations → Array methods (map, filter, reduce, etc.)
- Top-level let bindings and run expressions

CLI usage:
  lux compile app.lux --target js -o app.js
  lux compile app.lux --target js --run

Tested with factorial, datatypes, functional, tailcall, and hello
examples - all producing correct output when run in Node.js.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/codegen/mod.rs

@@ -2,10 +2,13 @@
 //!
 //! This module provides compilation to various targets:
 //! - C: For native compilation via GCC/Clang
-//! - JavaScript: For frontend/browser deployment (planned)
+//! - JavaScript: For frontend/browser deployment
 //! - WebAssembly: For portable deployment (planned)
 
 pub mod c_backend;
+pub mod js_backend;
 
 #[allow(unused_imports)]
 pub use c_backend::CBackend;
+#[allow(unused_imports)]
+pub use js_backend::JsBackend;