Dead code detection and tree-shaking

[brianp]

Two related features:

Dead code warnings (typechecker)
The typechecker already knows which symbols are referenced. Add a pass that walks from main(), builds a reachability graph, and warns on unreferenced top-level functions, classes, and imports. This includes methods on DynamicReceiver classes where match arms in method_missing align to known methods, those branches can be statically resolved and included in the call graph.

Tree-shaking (codegen)
Once the typechecker marks unreachable code, the FIR lowerer can skip emitting it entirely. For AOT builds this reduces binary size. For JIT it reduces compile time.