Improve recalc engine algorithm

(this is a bit more advanced for a first issue - requires knowledge of topological sorts. But it's marked as a "first issue" since the code is very accessible with few dependencies)

RecalcEngine's lets you define Formulas (via SetFormula) and then it will automatically recalc the formula if the dependencies change.

But the implementation here is extremely hack:
https://github.com/microsoft/Power-Fx/blob/e2ffacc79cd57a2f5b9a30cc05c41460d3478fb0/src/libraries/Microsoft.PowerFx.Interpreter/RecalcEngineWorker.cs#L18

This is the ability to call UpdateVariable() and cause other Formulas (created from SetFormula) to be recalculated.

It's good enough to pass the tests, but a very poor implementation.
See tests here:
https://github.com/microsoft/Power-Fx/blob/e2ffacc79cd57a2f5b9a30cc05c41460d3478fb0/src/tests/Microsoft.PowerFx.Interpreter.Tests/RecalcEngineTests.cs#L124

A good implementation:

• would statically understand what each formula depends on
• efficient recalc - does not unnecessary work.
• robust handling for circular references
• include aggressive tests.
• allow deleting a formula.

The current implementation is also very inefficient. We could construct a perf benchmark to show before and after.

Start with src/libraries/Microsoft.PowerFx.Interpreter/RecalcEngineWorker.cs and the RecalcEngineTests.cs cases around line 124. Compare the current behavior with the requested dependency analysis, efficient recalculation, circular-reference handling, deletion support, and broader tests; done means these requirements are covered and the existing tests still pass.