Allow `:preserve-case` on partial word match

[Ambrevar]

Case is not preserved if the matched term is not a whole word:

(ppcre:regex-replace-all "(?i)foo" "Fooz" "roo" :preserve-case t)
"rooz"
T

It works with whole words though:

> (ppcre:regex-replace-all "(?i)foo" "Foo z" "roo" :preserve-case t)
"Roo z"
T

This is by design, since string-case-modifier has this:

      (if (or (<= to from)
              (and (< start from)
                   (alphanumericp (char str (1- from)))
                   (alphanumericp (char str from)))
              (and (< to end)
                   (alphanumericp (char str to))
                   (alphanumericp (char str (1- to)))))
        ;; if it's a zero-length string or if words extend beyond FROM
        ;; or TO we return NIL, i.e. #'IDENTITY
        nil

My feature request is to add an option to allow preserving case even for words that extend beyond the boundaries.

What do you think?