* lisp/isearch.el (word-search-regexp): Match whitespace if the search

* lisp/isearch.el (word-search-regexp): Match whitespace if the search
string begins or ends in whitespace.  The LAX arg is applied to
both ends of the search string.  Use `regexp-quote' and explicit
\< and \> instead of \b.  Use \` and \' instead of ^ and $.
(isearch-symbol-regexp): Sync with `word-search-regexp' where word
boundaries are replaced with symbol boundaries, and characters
between symbols match non-word non-symbol syntax.

Fixes: debbugs:14602