Add CDP Node Registry

This expands on the existing CDP node work used in  DOM.search. It introduces
a node registry to track all nodes returned to the client and give lookups to
get a node from a Id or a *parser.node.

Eventually, the goal is to have the Registry emit the DOM.setChildNodes event
whenever necessary, as well as support many of the missing DOM actions.

Added tests to existing search handlers. Reworked search a little bit to avoid
some unnecessary allocations and to hook it into the registry.

The generated Node is currently incomplete. The parentId is missing, the
children are missing. Also, we still need to associate the v8 ObjectId to the
node.

Finally, I moved all action handlers into a nested "domain" folder.

src/main_unit_tests.zig

@@ -17,9 +17,9 @@
 // along with this program.  If not, see <https://www.gnu.org/licenses/>.
 
 const std = @import("std");
-const builtin = @import("builtin");
-const parser = @import("netsurf");
 const tls = @import("tls");
+const parser = @import("netsurf");
+const builtin = @import("builtin");
 
 const Allocator = std.mem.Allocator;
 
@@ -32,7 +32,6 @@ test {
     std.testing.refAllDecls(@import("css/match_test.zig"));
     std.testing.refAllDecls(@import("css/parser.zig"));
     std.testing.refAllDecls(@import("generate.zig"));
-    std.testing.refAllDecls(@import("http/client.zig"));
     std.testing.refAllDecls(@import("storage/storage.zig"));
     std.testing.refAllDecls(@import("storage/cookie.zig"));
     std.testing.refAllDecls(@import("iterator/iterator.zig"));