chore: refactor Execution and GlobalScope

The js.Execution is the API behind the Frame/WGS split, but the split is
actually held by the underlying js.GlobalScope. Most Execution methods are:

```zig
    return switch (self.js.global) {
        inline else => |g| g.isSameOrigin(url),
    };
```

And that works well, except that in some cases, code has a js.Context, not an
js.Execution, and they need to do the same inline switch.

This commit moves GlobalScope from src/browser/js to src/browser (there's
nothing JS/v8 about it), and moves all those inline switches into it. The
js.Execution API stays the same (it forwards the call to js.global)
but all callers that directly inlined switched the js.global no longer do.