Rename the delimited-scope entry points `%debug_this_sexp`, `%track_this_sexp` etc to "default" `%debug_sexp`, `%track_sexp`

[lukstafi]

Delimited scopes are less surprising. Do we need a good name for extended scopes e.g. %debug_below_sexp, %track_below_sexp, or can we get rid of them?