Switch to literal mode with message when regexp is too big in char-fold search

Switch to literal mode with message when regexp is too big in char-fold search

* lisp/char-fold.el (char-fold-to-regexp): Don't use regexp-quote
when the length of regexp reaches 5000.  (Bug#40216)

* lisp/isearch.el (isearch-search): On big regexp in char-fold mode
gracefully fall back to literal mode, try to search again and display
momentary-message about switching to literal mode.
(isearch--momentary-message): Add optional arg SECONDS.