Display matched offscreen open paren with a distinct face

Display matched offscreen open paren with a distinct face

Propertize matched offscreen openparen that is showing in
the echo area in order to make it prominent; use shadow
face for non-context characters (i.e., 'Matches') for the
same purpose.
* lisp/simple.el (blink-matching-paren-offscreen): Add this
face for highlighting.
* lisp/simple.el (blink-matching-paren-highlight-offscreen): Add
this option to toggle face `blink-matching-paren-offscreen'.
* lisp/simple.el (blink-paren-open-paren-line-string): Propertize
the matched offscreen openparen with a face conditionally.
(Bug#63089)