Add URL struct

Combine uri + rawuri into single struct.

Try to improve ownership around URIs and URI-like things.
 - cookie & request can take *const std.Uri
   (TODO: make them aware of the new URL struct?)
 - Location (web api) should own its URL (web api URL)
 - Window should own its Location

Most of these changes result in (a) a cleaner Page and (b) not having to carry
around 2 nullable objects (URI and rawuri).

src/cdp/cdp.zig

@@ -335,7 +335,7 @@ pub fn BrowserContext(comptime CDP_T: type) type {
 
         pub fn getURL(self: *const Self) ?[]const u8 {
             const page = self.session.currentPage() orelse return null;
-            return page.rawuri;
+            return if (page.url) |*url| url.raw else null;
         }
 
         pub fn onInspectorResponse(ctx: *anyopaque, _: u32, msg: []const u8) void {