patch 8.1.1693: syntax coloring and highlighting is in one big file

Problem:    Syntax coloring and highlighting is in one big file.
Solution:   Move the highlighting to a separate file. (Yegappan Lakshmanan,
            closes #4674)
diff --git a/src/Make_morph.mak b/src/Make_morph.mak
index 2565df5..6990a04 100644
--- a/src/Make_morph.mak
+++ b/src/Make_morph.mak
@@ -50,6 +50,7 @@
 	getchar.c						\
 	hardcopy.c						\
 	hashtab.c						\
+	highlight.c						\
 	indent.c						\
 	insexpand.c						\
 	json.c							\