Customizable char-fold with char-fold-symmetric, char-fold-include (bug#35689)

Customizable char-fold with char-fold-symmetric, char-fold-include (bug#35689)

* doc/emacs/search.texi (Lax Search): Document
char-fold-symmetric, char-fold-include, char-fold-exclude.

* lisp/char-fold.el (char-fold--default-include)
(char-fold--default-exclude, char-fold--default-symmetric)
(char-fold--previous): New defconsts.
(char-fold-include, char-fold-exclude, char-fold-symmetric):
New defcustoms.
(char-fold-make-table): Use them.
(char-fold-update-table): New function called at top-level.

* test/lisp/char-fold-tests.el (char-fold--test-no-match-exactly)
(char-fold--permutation): New functions.
(char-fold--test-without-customization)
(char-fold--test-with-customization): New tests.