Placeholder for fold
Importance: Minor
Description: Add an optional placeholder argument to the fold function, i.e. change the signature to fold(regions, <placeholder>) and fold([regions], <placeholder>). If the function is executed the placeholder text is inside the yellow rectangle instead of the 3 dots.
In addition add a function is_folded(point), which returns whether a point is currently folded.
Motivation: This could be used to fold redundant and unnecessary code away to improve readability while keeping the key information. E.g. fold all function keywords in JavaScript to f or python lambda: to λ. In LaTeX it could be used to fold labels to l and greek characters and some operators to their unicode character, e.g. \alpha to α.
