patch 8.2.4911: the mode #defines are not clearly named

Problem:    The mode #defines are not clearly named.
Solution:   Prepend MODE_.  Renumber them to put the mapped modes first.
diff --git a/src/search.c b/src/search.c
index 3165ba2..c32475c 100644
--- a/src/search.c
+++ b/src/search.c
@@ -2860,7 +2860,7 @@
 	    save_dollar_vcol = dollar_vcol;
 #ifdef CURSOR_SHAPE
 	    save_state = State;
-	    State = SHOWMATCH;
+	    State = MODE_SHOWMATCH;
 	    ui_cursor_shape();		// may show different cursor shape
 #endif
 	    curwin->w_cursor = mpos;	// move to matching char