Improve speed of the Folding Provider

[glennsarti]

In Pull Request #777 a syntax aware folder was added to Editor Services. During the review if the PR it was found there are quite a few performance improvements that could be made to reduce execution / resource IO as the folding provider is called quite often.

This issue will track the work to make the folding provider more streamline and faster.

Why didn't we do this at first? The implementation in the initial PR is good enough and for small or medium size PS files it responds in an adequate timeframe. Also the release cadence of PSES allows us to get functionality in quickly and then refactor.

Relates to #793