feat: allow direct base64 call against HandleValue

[guybedford]

Adds back the ability to call convertJSValueToByteString for a HandleValue by declaring the header for the original function variant. Necessary for Fastly's statusText setter to pass the web platform tests, because converting to a string first was altering unsupported characters.

[tschneidereit]

I hadn't really looked at this code before. Obviously adding this declaration makes sense, and I'm okay with landing this as-is.

Ideally though, it'd be great to rename at least the other export, or perhaps both. I think something like ToJSByteString would work well for both.

Also, it took me a bit to understand what this does: I first thought it'd base64 encode the input, where really what it does is coerce the input value to a latin1 string iff the result of stringifying it allows for that to happen losslessly. Having a comment explaining that would be great.

However, none of that needs to block landing this if you'd prefer getting on with things.

[guybedford]

I've renamed these to valueToJSByteString and stringToJSByteString respectively, let me know if that works for you.

[tschneidereit]

Sure, yeah. I think we could give them the same name and rely on overloading resolution, but this works, too.