Move more asserts to custom asserter.

Deciding what should be an lp.assert, vs an std.debug.assert, vs a debug-only
assert is a little arbitrary.

debug-only asserts, guarded with an `if (comptime IS_DEBUG)` obviously avoid the
check in release and thus have a performance advantage. We also use them at
library boundaries. If libcurl says it will always emit a header line with a
trailing \r\n, is that really a check we need to do in production? I don't think
so. First, that code path is checked _a lot_ in debug. Second, it feels a bit
like we're testing libcurl (in production!)..why? A debug-only assertion should
be good enough to catch any changes in libcurl.

src/Notification.zig

@@ -17,6 +17,7 @@
 // along with this program.  If not, see <https://www.gnu.org/licenses/>.
 
 const std = @import("std");
+const lp = @import("lightpanda");
 
 const log = @import("log.zig");
 const Page = @import("browser/Page.zig");
@@ -241,7 +242,7 @@ pub fn unregister(self: *Notification, comptime event: EventType, receiver: anyt
     if (listeners.items.len == 0) {
         listeners.deinit(self.allocator);
         const removed = self.listeners.remove(@intFromPtr(receiver));
-        std.debug.assert(removed == true);
+        lp.assert(removed == true, "Notification.unregister", .{ .type = event });
     }
 }