Implement RandomAccessCollection on JSArrayRef

[MaxDesiatov]

This allows using Int subscripts on JSArrayRef instances and makes the rest of the functions available from the relevant extensions. This may shadow some JavaScript functions with the same names (I don't remember any off the top of my head), but I hope that the added convenience is worth it.

Also .gitignore is amended to include the .build directory created by make test.

[j-f1]

Great! The reason I went so far with the JSObjectRef is because an object typically has a bunch of properties and I wanted them to be accessible. Since arrays usually don’t have additional properties, I think that in the rare case there’s a conflict, it’s ok to have to use the subscript syntax.

[MaxDesiatov]

After having another look, JSArrayRef doesn't have dynamic member subscripts, so none of the JS properties or functions are exposed on it, no chance of collision then. If a user needs to access those they should just cast that object to JSObjectRef through the .object property of JSValue instead of .array.