typings: add ffi internal binding types

[HoonDongKang]

Add typings for the ffi internal binding.

Changes:

• Added typings/internalBinding/ffi.d.ts
• Typed DynamicLibrary and its methods
• Typed native memory access and pointer helpers
• Typed internal FFI constants and symbols

[panva]

cc @nodejs/ffi are these typings accurate? will you remember to keep the typings up to date? it's undergoing very active development so might as well be the case that in-flight changes may invalidate this very proposed state.

[HoonDongKang]

Thanks for raising this. I'm aware that the FFI module is under active development, which I considered before opening this PR. I decided to type the current implementation because having typings—even at this early stage-significantly improves the developer experience.

That said, would you prefer holding off on merging these until the API stabilizes further?

[ShogunPanda]

@HoonDongKang No, I think we need a starting point anyway. And lately the FFI API has been pretty stable TBH.

[HoonDongKang]

Thank you! I'll periodically check for updates and continue contributing to keep these typings up to date. :)