Allow matching any wall map terrain in lua scripts

When matching a terrain, allow using a "w" placeholder that matches
any solid wall:

For example:
   local s = selection.match([[w.w]]);
would match all floor locations with a wall to the left and right of it.
The walls can be solid stone, horizontal, vertical, etc.

This applies to selection.match(), selection.filter_mapchar(), and
des.replace_terrain()

src/nhlua.c

@@ -206,6 +206,7 @@ const struct {
                 { 'F', IRONBARS }, /* Fe = iron */
                 { 'x', MAX_TYPE }, /* "see-through" */
                 { 'B', CROSSWALL }, /* hack: boundary location */
+                { 'w', MATCH_WALL }, /* IS_STWALL() */
                 { '\0', STONE },
 };