Reorganize v8 contexts and scope

- Pages within the same session have proper isolation
  - they have their own window
  - they have their own SessionState
  - they have their own v8.Context

- Move inspector to CDP browser context
  - Browser now knows nothing about the inspector

- Use notification to emit a context-created message
  - This is still a bit hacky, but again, it decouples browser from CDP

src/url.zig

@@ -8,6 +8,8 @@ pub const URL = struct {
     uri: Uri,
     raw: []const u8,
 
+    pub const empty = URL{ .uri = .{ .scheme = "" }, .raw = "" };
+
     // We assume str will last as long as the URL
     // In some cases, this is safe to do, because we know the URL is short lived.
     // In most cases though, we assume the caller will just dupe the string URL