feat: add Std.Broadcast type (#10369)

This PR adds a multi-consumer, multi-producer channel to Std.Sync.

This PR depends on: #10366, #10367 and #10370.

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Co-authored-by: Markus Himmel <[email protected]>

Author: algebraic-dev

src/Std/Sync.lean

@@ -13,3 +13,6 @@ public import Std.Sync.RecursiveMutex
 public import Std.Sync.Barrier
 public import Std.Sync.SharedMutex
 public import Std.Sync.Notify
+public import Std.Sync.Broadcast
+
+@[expose] public section