Make Callback.printFunc public

[karlseguin]

When calling a Zig function from JS fails due to a parameter type error, log.debug information about the function and parameters.

[sjorsdonkers]

How do I get this to print?
When I add an argument (to IntersectionObserver.observer()), I get nothing in the server terminal and the same invalid argument response to the client.

[karlseguin]

It's a debugging tool, in case you're getting some JS error in a callback, and want to see what that callback is.

    pub fn constructor(callback: Env.Callback, options_: ?IntersectionObserverOptions, state: *SessionState) !IntersectionObserver {
        // ADDED
        try callback.printFunc();
        ....

$ make test F="Browser.DOM.IntersectionObserver"
() => {}
entries => {count_a += 1;}
entries => {count_b = entries.length;}
entries => {count_bb = entries.length;}
entries => { count_c = entries.length;}
entries => {}
entries => {}
entries => { entry = entries[0]; }
entries => { new_entry = entries[0]; }
Browser.DOM.IntersectionObserver (13.87ms)

[sjorsdonkers]

Ah, got it.
We could look into always calling this when we have an invalid arguments error on a callback (in debug)