feat: add A* search algorithm

[InukaWijerathna]

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.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.97%. Comparing base (5c39e87) to head (645f14d).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1909      +/-   ##
==========================================
+ Coverage   85.91%   85.97%   +0.05%     
==========================================
  Files         379      380       +1     
  Lines       19778    19856      +78     
  Branches     3016     3026      +10     
==========================================
+ Hits        16993    17071      +78     
  Misses       2785     2785              

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