Introduce Task::Queue

- This patch implements `Task::Queue` mirroring `Thread::Queue` in CRuby.
- Producer/Consumer pattern is now possible with no polling

## No Top-level pollution
- No top-level `Queue` defined
- No `TaskError`. Instead, `Task::Error` happens when `Task::Queue#pop(true)` when empty
- No `ClosedQueueError`. `Task::Error` also happens when pushing to closed queue

## Future Work
- `Task::SizedQueue`

mrbgems/mruby-task/examples/queue.rb

@@ -0,0 +1,54 @@
+# Task::Queue Example
+# Demonstrates producer/consumer coordination using Task::Queue.
+# No polling required - consumers block until items are available.
+
+puts "=== Task::Queue Producer/Consumer Demo ==="
+puts
+
+TOTAL_ITEMS = 10
+q = Task::Queue.new
+produced = 0
+consumed = 0
+
+producer = Task.new(name: "producer") do
+  TOTAL_ITEMS.times do |i|
+    item = "item-#{i}"
+    q.push(item)
+    produced += 1
+    puts "Producer: pushed #{item}"
+    sleep 0.1
+  end
+  q.close
+  puts "Producer: closed queue after #{produced} items"
+end
+
+consumer1 = Task.new(name: "consumer-1") do
+  loop do
+    item = q.pop  # blocks until an item is available or queue closes
+    break if item.nil?
+    consumed += 1
+    puts "Consumer-1: got #{item}"
+    sleep 0.15
+  end
+  puts "Consumer-1: done"
+end
+
+consumer2 = Task.new(name: "consumer-2") do
+  loop do
+    item = q.pop
+    break if item.nil?
+    consumed += 1
+    puts "Consumer-2: got #{item}"
+    sleep 0.2
+  end
+  puts "Consumer-2: done"
+end
+
+Task.run
+
+puts
+puts "=== Summary ==="
+puts "Produced: #{produced}"
+puts "Consumed: #{consumed}"
+puts "Queue size: #{q.size}"
+puts "Queue closed: #{q.closed?}"