Add customizable behavior allowing backward-delete of blank sexps

[timor]

This allows deleting empty sexps not only when they are completely empty (closing delimiter follows immediately after opening delimiter), but also if they only consist of white-space.

This is useful for modes where white-space is mandatory after/before delimiters, to get the same behavior for deleting empty sexps.

This also changes the return value of sp-point-in-blank-sexp to
return the beginning and ending positions of the enclosing regexp.

[Fuco1]

Should we also want this in the situation where ( |) and we hit C-d? That is the behaviour in strict mode that works with (|).

[timor]

Should we also want this in the situation where ( |) and we hit C-d? That is the behaviour in strict mode that works with (|).

I thought about that, but since I never had that use case, I did not implement it. Should be straightforward, though. Shall I add it?