Fix calculator-string-to-number yet again (bug#47694)

Fix calculator-string-to-number yet again (bug#47694)

* lisp/calculator.el (calculator-string-to-number):
The last bugfix changed the code to just blindly replace ".e".  This
has some minor problems like making "-." parse as 0.0 instead of -0.0,
and ".1.e1" is parsed as 1 instead of 0.1.  Instead, replace the first
"." that is followed by a non-digit with ".0".  Since this has had
several problems over the years, add some tests too.  (Also, restore
the original if-indentation style.)