Change how we integrate with case-fold-search

[Haxxflaxx]

I've been looking a bit at integration between isearch and general meow functionality and I have settled on two rather simple hooks that for the most part does what I want it to do:

(defun da/meow-after-isearch-integration ()
    (when (and (or meow-normal-mode meow-beacon-mode)
               (not isearch-mode-end-hook-quit))
      (thread-first
            (meow--make-selection '(select . visit) isearch-other-end (point))
            (meow--select t))
        (when meow-beacon-mode
          (let ((select-string (if isearch-regexp isearch-string
                                 (buffer-substring-no-properties (region-beginning) (region-end)))))
            (meow--push-search select-string)
            (meow--highlight-regexp-in-buffer select-string)))))

(add-hook 'isearch-mode-hook (lambda () (ignore-errors (meow-cancel-selection))))
(add-hook 'isearch-mode-end-hook 'da/meow-after-isearch-integration)

The only real issue with it is in beacon-mode where the differences in how case-fold-search is handled.
In meow we explicitly set it to nil when doing searches, which for the general use case of meow would be the correct option, but the way it is set in a let today makes it so that it can't be manipulated when needed.

I propose we introduce something like a meow-case-fold-search which would be used in the meow related searches. The usual entry points we have e.g. meow-mark-* and meow-visit would set this variable to nil and get the same behavior as we have today.
We could maybe also have something like meow-toggle-case-fold or something, that toggles the variable and updates any overlays as applicable.

With something like this it should be easier to do integrations between search functionalities.