patch 8.0.1777: cannot cleanup before loading another colorscheme

Problem:    Cannot cleanup before loading another colorscheme.
Solution:   Add the ColorSchemePre autocommand event.
diff --git a/src/vim.h b/src/vim.h
index af01f1a..0a4a490 100644
--- a/src/vim.h
+++ b/src/vim.h
@@ -1277,6 +1277,7 @@
     EVENT_CMDWINENTER,		/* after entering the cmdline window */
     EVENT_CMDWINLEAVE,		/* before leaving the cmdline window */
     EVENT_COLORSCHEME,		/* after loading a colorscheme */
+    EVENT_COLORSCHEMEPRE,	/* before loading a colorscheme */
     EVENT_COMPLETEDONE,		/* after finishing insert complete */
     EVENT_CURSORHOLD,		/* cursor in same position for a while */
     EVENT_CURSORHOLDI,		/* idem, in Insert mode */