populate Response.statusText for network responses

Fetch responses from the network now have their statusText set using
the HTTP status phrase (e.g. "OK", "Not Found"), matching the Fetch
spec and the existing XMLHttpRequest behavior.

src/browser/webapi/net/Fetch.zig

@@ -117,6 +117,7 @@ fn httpHeaderDoneCallback(transfer: *Http.Transfer) !bool {
     }
 
     res._status = header.status;
+    res._status_text = std.http.Status.phrase(@enumFromInt(header.status)) orelse "";
     res._url = try arena.dupeZ(u8, std.mem.span(header.url));
     res._is_redirected = header.redirect_count > 0;