feat: tree-shake unused extension free functions from export

[malec-palec]

• Only include lifecycle function .js files (onFirstSceneLoaded, onSceneLoaded, etc.) as mandatory extension-wide includes.
• Non-lifecycle free functions are now included only when actually referenced by event sheets.

Previously, using any single free function (or behavior/object) from an extension pulled ALL free function .js files into the build via extensionIncludeFiles. This was a conservative safety net that is redundant - the code generation pass already records transitive include dependencies per-function.

Why this is safe

1. Transitive deps already handled - when function A's events call function B, the code generator adds B's include to A's metadata during the second-pass compilation.
2. Lifecycle functions preserved - onFirstSceneLoaded, onScenePreEvents, etc. self-register via gdjs._registerCallback() at load time and are never referenced through event instructions; they remain in the mandatory include list.
3. Behaviors/objects unaffected - their .js is included via their own metadata when the type is present in the project.
4. Validated empirically - static analysis of multiple production projects confirmed that top-level free functions have no implicit invocation paths beyond event sheet instructions and lifecycle registration.