Refactor CDP

CDP is now an struct which contains its own state a browser and a session.

When a client connection is made and successfully upgrades, the client creates
the CDP instance. There is now a cleaner separation betwen Server, Client and
CDP.

Removed a number of allocations, especially when writing results/events from
CDP to the client. Improved input message parsing. Tried to remove some usage
of undefined.

src/main.zig

@@ -261,12 +261,13 @@ pub fn main() !void {
             defer loop.deinit();
 
             // browser
-            var browser = Browser{};
-            try Browser.init(&browser, alloc, &loop, vm);
+            var browser = Browser.init(alloc, &loop);
             defer browser.deinit();
 
+            var session = try browser.newSession({});
+
             // page
-            const page = try browser.session.createPage();
+            const page = try session.createPage();
             try page.start(null);
             defer page.end();