Enhancement to let related refactor functions

Replaces all occurrences of the init expression (the sexp which is moved to let for example)

- feature is triggered when let is expanded and replaces occurrences of init expression in newly enfolded sexp
- feature is triggered when new sexp is moved to let when user exits multiple cursors mode

## Known limitations

## Possible improvements
- for moving to let it would be possible to do one instance of a fake cursor for each occurrence of the init expression instead of doing the replacing after user exists multiple cursors mode