patch 8.2.3139: functions for string manipulation are spread out

Problem:    Functions for string manipulation are spread out.
Solution:   Move string related functions to a new source file. (Yegappan
            Lakshmanan, closes #8470)
diff --git a/src/Make_cyg_ming.mak b/src/Make_cyg_ming.mak
index e93165d..f12ffac 100644
--- a/src/Make_cyg_ming.mak
+++ b/src/Make_cyg_ming.mak
@@ -809,6 +809,7 @@
 	$(OUTDIR)/spell.o \
 	$(OUTDIR)/spellfile.o \
 	$(OUTDIR)/spellsuggest.o \
+	$(OUTDIR)/strings.o \
 	$(OUTDIR)/syntax.o \
 	$(OUTDIR)/tag.o \
 	$(OUTDIR)/term.o \