webapi: resource-timing

Adds resource timing, e.g. `performance.getEntriesByType("resource")`.

The `resource-timing` WPT category is currently at 4.6%, and this is a first
step at improving it. It also hopefully fixes https://github.com/lightpanda-io/browser/issues/3359

This is more complicated than I thought because there's a "Timing-Allow-Origin"
header that a server can include which hides some of the data if the request
doesn't come from the listed origin. And that, of course, interacts with
redirects.

(The DOMException change is seemingly random, but it came up in one of the WPT
cases I was looking at).

src/testing.zig

@@ -693,6 +693,33 @@ fn testHTTPHandler(req: *std.http.Server.Request) !void {
         });
     }
 
+    // Scripts, so a cross-origin (localhost) load doesn't depend on CORS.
+    if (std.mem.eql(u8, path, "/resource-timing/tao")) {
+        return req.respond("window.__rt_tao = true;", .{
+            .extra_headers = &.{
+                .{ .name = "Content-Type", .value = "text/javascript" },
+                .{ .name = "Timing-Allow-Origin", .value = "https://example.com, *" },
+            },
+        });
+    }
+
+    if (std.mem.eql(u8, path, "/resource-timing/plain")) {
+        return req.respond("window.__rt_plain = true;", .{
+            .extra_headers = &.{
+                .{ .name = "Content-Type", .value = "text/javascript" },
+            },
+        });
+    }
+
+    if (std.mem.eql(u8, path, "/resource-timing/redirect")) {
+        return req.respond("", .{
+            .status = .found,
+            .extra_headers = &.{
+                .{ .name = "Location", .value = "/resource-timing/plain" },
+            },
+        });
+    }
+
     if (std.mem.eql(u8, path, "/redirect-no-fragment")) {
         return req.respond("", .{
             .status = .found,