Summary:
This pull request makes two minor changes to `jsi.h`:
* Tweak the `JSI_EXPORT` macro to automatically set itself to an empty value if `_MSC_VER` is defined - like how was done by acoates-ms [here](8beb4bb58a/ReactCommon/cxxreact/JSBigString.h (L15-L21)).
* Tweak the call to constructor `Pointer(Runtime::PointerValue* ptr)` in the constructor for `PropNameID`. I am not sure why MSVC wasn't working with the original version, but it compiles after I tweak that.
[General] [Fixed] - Tweaked `jsi.h` to build on MSVC
Pull Request resolved: https://github.com/facebook/react-native/pull/23367
Differential Revision: D14032507
Pulled By: cpojer
fbshipit-source-id: 701c13e3509cc244dbe0c15f92067fae4382bee2