Fix deadlock issue with example code in wasm-heif README.md

[SwimmingTiger]

I spent a long time debugging the sample code at npm:@saschazar/wasm-heif. It just deadlocks in heifModule.decode() without any output.

In the end I found out that this is because we cannot call heifModule.decode() in onRuntimeInitialized(). If I move it outside, everything works fine.