Fixes StringBuilder unbounded size growth in Clear() when we use a mix of Append...

Fixes StringBuilder unbounded size growth in Clear() when we use a mix of Append and Insert (#16926)

* Fixing Clear infinitely growing when we combine usage of Insert and Append. Fixes #27625

* Move debug code to StringBuilder.Debug.cs and Applied code review feedback

* Adding missing debug condition check

* Adding comments and moving Condition on projitems after filename

* Moving the infinite capacity growth fix to Length setter

* Removing originalCapacity and Debug.Assert

* Applying PR feedbacks

* Minor cleanup

* simplifying to single loop

* keeping just one method for ShowChunks function