feat: New Callback implementation that extends () -> T in Kotlin (faster!)

[mrousavy]

This PR changes the way how functions work in Kotlin/Android.
Previously we only had native functions that can be passed to Kotlin.
Now we can also have actual Kotlin functions (lambdas) that can be passed to C++.

So for a Func_void, we now have

• JFunc_void: base interface with an invoke() method
• JFunc_void_cxx: The C++-based implementation that holds a std::function<void()> - most likely from a jsi::Function (JNI)
• JFunc_void_java: The Java based implementation that holds a () -> Unit, which can be called from C++ using JNI

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Also, instead of explicitly converting a Func_void to a () -> Unit (toLambda()), it now no longer needs to be converted to one since it always is one. The Java interface Func_void **inherits from () -> Unit.

fun interface Func_void: () -> Unit {
  //                     ^ cool part
  override fun invoke() {
    // ...
  }
}

This way Func_void is always a callable.

---

Also I added these two tests to ensure this won't break again:

• optionalCallback?: (number) => void test (callback property, instead of method)
• complexReturn(): T to harden requiresSpecialBridging for Kotlin

---

This PR fixes a few bugs around callbacks and structs, e.g.:

• Structs that hold a Callback couldn't be passed from JS -> C++ -> Kotlin
• Callbacks couldn't be passed from Kotlin -> C++ -> JS

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