bug#20351: isearch-lax-whitespace should be customizable variable

[Juri Linkov]

>> ‘case-fold-search’ is a customizable variable - and rightfully so
>> ‘char-fold-search’ has to be customizable as well.
>
> Yes, we should make it a defcustom before release. I didn't make it a
> defcustom when I implementing it because the implementation itself is
> still up for debate.

What do you think about using the shorter name ‘char-fold-search’?
All char-related functions in Emacs are shortened to “char”, e.g.
‘char-code’, ‘char-table’, ‘char-to-string’, etc.