Improve frame sub-navigation

This makes frame sub-navigation "work" for all page navigations (click, form
submit, location.top...) as well as setting the iframe.src.

Fixes at least 2 WPT crashes.

BUT, the implementation still isn't 100% correct, with two known issues:

1. Navigation currently happens in the context where it's called, not the
   context of the frame. So if Page1 accesses Frame1 and causes it to navigate,
   e.g. f1.contentDocument.querySelector('#link').click(), it's Page1 that will
   be navigated, since the JS is being executed in the Page1 context.
   This should be relatively easy to fix.

2. There are particularly complicated cases in WPT where a frame is navigated
   inside of its own load, creating an endless loop. There's some partial
   support for this as-is, but it doesn't work correctly and it currently is
   defensive and likely will not continue to navigate. This is particularly true
   when sub-navigation is done to about:blank within the frame's on load event.
   (Which is probably not a real concern, but an issue for some WPT tests)

Although it shares a lot with the original navigation code, there are many more
edge cases here, possibly due to being developed along side WPT tests. The
source of most of the complexity is the synchronous handling of "about:blank"
in page.navigate, which can result in a scheduled navigation synchronously
causing more scheduled navigation. (Specifically because
`self.documentIsComplete();` is called from page.navigate in that case). It
might be worth seeing if something can be done about that, to simplify this new
code (removing the double queue, removing the flag, simplifying pre-existing
schedule checks ,...)

src/browser/js/Function.zig

@@ -160,8 +160,8 @@ fn _tryCallWithThis(self: *const Function, comptime T: type, this: anytype, args
             try_catch.rethrow();
             return error.TryCatchRethrow;
         }
-        caught.* = try_catch.caughtOrError(local.call_arena, error.JSExecCallback);
-        return error.JSExecCallback;
+        caught.* = try_catch.caughtOrError(local.call_arena, error.JsException);
+        return error.JsException;
     };
 
     if (@typeInfo(T) == .void) {