Add implementation of StateMachine utility

Claude-generated.

Signed-off-by: James M Snell <[email protected]>

Author: jasnell

src/workerd/util/BUILD.bazel

@@ -387,3 +387,15 @@ kj_test(
     src = "small-set-test.c++",
     deps = [":small-set"],
 )
+
+wd_cc_library(
+    name = "state-machine",
+    hdrs = ["state-machine.h"],
+    visibility = ["//visibility:public"],
+    deps = ["@capnp-cpp//src/kj"],
+)
+
+kj_test(
+    src = "state-machine-test.c++",
+    deps = [":state-machine"],
+)