doctest_collect.m: Improved indentation and empty lines in texinfo extraction
doctest_collect.m: Improved indentation and empty lines in texinfo extraction

* Adding a newline at the beginning of the texinfo code leads to
  inconsistent indentation of example groups after conversion to
  plain text via makeinfo under Windows if all texinfo lines are indented.
  We only need to add the additional newline in files that have content
  in the first line and have no indentation.

* Only search for spaces and tabs as indenting characters. \s also matches
  newline characters and would add additional empty lines at the beginning
  and at the end of example blocks.

* When removing the EXAMPLE markers, remove the whole line and do not
  retain empty lines at the beginning of example blocks.
  But keep the final newline character, because this is needed during
  doctest_run.

catch22/octave-doctest