css: allow escaped first char in identifier name

src/runtime/test_primitive_types.zig

@@ -336,4 +336,8 @@ test "JS: primitive types" {
         .{ "p.returnFloat32()", "1.100000023841858,-200.03500366210938,0.0003000000142492354" },
         .{ "p.returnFloat64()", "8881.22284,-4928.3838122,-0.00004" },
     }, .{});
+
+    try runner.testCases(&.{
+        .{ "'foo\\\\:bar'", "foo\\:bar" },
+    }, .{});
 }