Restructure modules with per-module addon subdirs

- Each module (outline, interact, screenshot) now has its own addon/
  subdir with .toc, .lua, and Bindings.xml
- Core WeirdUtils addon lives in src/core/addon/
- build.zig supports compile-time feature gating via -D options
- main.zig uses build_options for conditional imports and addon embedding
- Module addons only load when their module is compiled in
- 'zig build all-variants' produces full.dll + 3 single-module DLLs

src/interact/addon/Interact.toc

@@ -0,0 +1,8 @@
+## Interface: 11200
+## Title: Interact
+## Notes: Nearest NPC/object interaction and bulk looting
+## Version: 1.0
+## Dependencies: WeirdUtils
+
+Interact.lua
+Bindings.xml