[DRAFT] Fix JSCRuntime::createStringFromUtf8 to support strings with embedded null chars

[zagriswo]

DRAFT - Do not merge

Please select one of the following

• I am removing an existing difference between facebook/react-native and microsoft/react-native-macos 👍
• I am cherry-picking a change from Facebook's react-native into microsoft/react-native-macos 👍
• I am making a fix / change for the macOS implementation of react-native
• I am making a change required for Microsoft usage of react-native

Summary

The current implementation of JSCRuntime::createStringFromUtf8 ignores the length parameter that it's given, and then blindly passes the char array to JSStringCreateWithUTF8CString which assumes the char array is a null terminated string. This means we can't pass strings with embedded null chars through the JSI boundary from C++ to JS. Instead, we can internally convert the string from UTF-8 to UTF-16, and then call JSStringCreateWithCharacters which allows us to pass an explicit length for our array of chars.

JSStringCreateWithUTF8CString internally has an optimization for strings that only contain ASCII characters, which we will miss out on by switching to JSStringCreateWithCharacters. The JSC API surface isn't rich enough for us to avoid that, so if there's a perf impact here, then so be it. We will trade perf for correctness.

This is based on PR 34300 upstream in the RN repo. I'm not assuming infinite flexibility in this version. We're only building for Apple platforms, so we know the endianness and the expected size of a JSChar.

[zagriswo]

@Saadnajmi - draft from our conversation

[zagriswo]

Abandoned due to the perf impact of this change.