Introduce a "transfer_arena"

Some data has to exist specifically for the navigation of one page to another.
For example, if a hyperlink is clicked, the URL begins its life with the
original page, but is transferred to the new page. The page_arena cannot be used
for such data.

It's possible to use the session_arena, but it's lifetime is much longer and,
given enough navigation, could accumulate a lot of memory.

The new transfer_arena exists within the session, but only exists until the
next navigation.

While currently only used for the navigation URL, the main goal here is to have
a place to put the request body on form submission, which has a lifetime similar
to a click url.

While I'm at it, I promoted the existing session arena and the new transfer
arena to the browser, allowing better memory re-use between sessions.

src/cdp/cdp.zig

@@ -314,7 +314,7 @@ pub fn BrowserContext(comptime CDP_T: type) type {
             const allocator = cdp.allocator;
 
             const session = try cdp.browser.newSession();
-            const arena = session.arena.allocator();
+            const arena = session.arena;
 
             const inspector = try cdp.browser.env.newInspector(arena, self);