feat(WASM): implement playback observer callbacks

[theashraf]

Problem:

• dotLottie-rs WASM bindings are generated from the uniffi cpp bindings using emscripten.
• uniffi cpp depends on an older version (0.25) of uniffi that lacks trait bindings, making it unfeasible to provide WASM bindings for the dotLottie observer.

Changes:

• Introduce the wasm_observer_callbacks_ffi module a stable C-ABI interface for observer callbacks for wasm32 target
• Provide a custom JavaScript implementation for wasm_observer_callbacks_ffi via emscripten_library.js using the --js-library flag.

This approach serves as an "escape hatch," allowing us to unblock state machine event integrations in the WASM environment. The trade-off is that we can no longer apply the closure compiler for minimized bundle sizes at this stage

[changeset-bot]

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