vim-patch:8.2.4911: the mode #defines are not clearly named (#18499)

Problem:    The mode #defines are not clearly named.
Solution:   Prepend MODE_.  Renumber them to put the mapped modes first.
249591057b

A hunk from the patch depends on patch 8.2.4861, which hasn't been
ported yet, but that should be easy to notice.

src/nvim/search.c

@@ -2428,7 +2428,7 @@ void showmatch(int c)
 
       save_dollar_vcol = dollar_vcol;
       save_state = State;
-      State = SHOWMATCH;
+      State = MODE_SHOWMATCH;
       ui_cursor_shape();                // may show different cursor shape
       curwin->w_cursor = mpos;          // move to matching char
       *so = 0;                          // don't use 'scrolloff' here