The Graphs folder already has several shortest-path algorithms (Dijkstra, Bellman-Ford, Floyd-Warshall) but no heuristic-guided search. This adds the A* search algorithm, which generally explores fewer nodes than Dijkstra's by using a heuristic to steer toward the target.

Implementation notes:

• Reuses the existing KeyPriorityQueue (Data-Structures/Heap/KeyPriorityQueue.js) for the open set instead of introducing a new heap.
• Takes a general weighted adjacency-list graph ({ node: [[neighbor, weight], ...] }) and a heuristic(node) function, consistent with how other graph algorithms here take adjacency-list/edge-list input.
• Passing a heuristic that always returns 0 makes it behave exactly like Dijkstra's algorithm, which is covered by a test.
• Returns { path, cost } for the shortest path, or null if the target is unreachable.

Test plan

• Graphs/test/AStar.test.js covers: a weighted graph with an admissible heuristic, the zero-heuristic/Dijkstra-equivalent case, start === target, and an unreachable target.
• Ran the full suite locally (npm test): 347 test files, 11,975 tests passing.
• npm run check-style passes.