feat: Add jsi::ArrayBuffer::getMutableBuffer()

[mrousavy]

Summary

Adds a new API to jsi::ArrayBuffer: getMutableBuffer() - which returns a std::shared_ptr<MutableBuffer> if it carries one (external buffer, held in InternalPropertyArrayBufferExternalFinalizer), or nullptr otherwise.

I needed this in Nitro to efficiently pass native buffers between JS and native, and back.

Before this API, I lost all native buffer information if it was passed native -> JS -> native again.

• Fixes Support for ArrayBuffer::isMutableBuffer() and ArrayBuffer::getMutableBuffer() #1578

Test Plan

(haven't tested this yet, trying to figure out how you guys run tests in hermes)

[mrousavy]

sry for the messy commit history, I was testing in CI until I figured out how to build the test suite locally, lol

[mrousavy]

@tmikov do we need an implementation for HermesABIRuntimeWrapper::getMutableBuffer(...)?

[erights]

Why not just implement the Immutable ArrayBuffers proposal?

[mrousavy]

Why not just implement the Immutable ArrayBuffers proposal?

1. This doesn't have anything to do with what I'm implementing. I implemented a native API to get a native buffer. You're talking about JS APIs.
2. The linked proposal is not even yet final, and not implemented in V8 nor JSC.

[erights]

This doesn't have anything to do with what I'm implementing. I implemented a native API to get a native buffer. You're talking about JS APIs

Thanks for the clarification. I had not understood that.