feat: add Chapter 2 gold miner FSM example in Go

Port the finite state machine from "Programming Game AI by Example"
chapter 2: BaseGameEntity, a generic State[T]/StateMachine[T] pair
(Go generics standing in for the book's C++ templates), and a Miner
that cycles through digging, banking, sleeping, and drinking states.
VisitBankAndDepositGold, GoHomeAndSleepTilRested, and QuenchThirst
are original implementations based on the book's written behavior
description, since their C++ source wasn't available. Includes a
runnable demo in main.go.

chapter-2-state-driven-agent-design/main.go

@@ -0,0 +1,22 @@
+// Command westworld runs a tiny demo of the finite state machine
+// built in the westworld package: it creates one miner and lets him
+// tick forward on his own, watching his state machine drive him
+// around the loop (sleep -> dig -> bank/saloon -> sleep, ...).
+package main
+
+import (
+	"fmt"
+
+	"git.jjstudio.me/jonas/Programming-Game-AI-by-Example-Go/chapter-2-state-driven-agent-design/westworld"
+)
+
+func main() {
+	bob := westworld.NewMiner(1)
+
+	const numUpdateSteps = 30
+	for range numUpdateSteps {
+		bob.Update()
+	}
+
+	fmt.Println()
+}