New minor mode 'minibuffer-regexp-mode' for editing regexps in minibuffer

This highlights parens via `show-paren-mode' and
`blink-matching-paren' in a user-friendly way, avoids reporting
alleged paren mismatches and makes sexp navigation more
intuitive.

* lisp/minibuffer.el (minibuffer-regexp-mode)
(minibuffer--regexp-primed, minibuffer--regexp-prompt-regexp):
New variables.
(minibuffer--regexp-propertize, minibuffer--regexp-primed)
(minibuffer--regexp-before-change)
(minibuffer--regexp-after-change)
(minibuffer--regexp-post-self-insert, minibuffer--regexp-setup)
(minibuffer--regexp-exit, minibuffer-regexp-mode): New functions.
(minibuffer-regexp-prompts): New option.