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_mvc.mak b/src/Make_mvc.mak
index 4d6e49b..8c8e71a 100644
--- a/src/Make_mvc.mak
+++ b/src/Make_mvc.mak
@@ -819,6 +819,7 @@
 	$(OUTDIR)\spell.obj \
 	$(OUTDIR)\spellfile.obj \
 	$(OUTDIR)\spellsuggest.obj \
+	$(OUTDIR)\strings.obj \
 	$(OUTDIR)\syntax.obj \
 	$(OUTDIR)\tag.obj \
 	$(OUTDIR)\term.obj \
@@ -1792,6 +1793,8 @@
 
 $(OUTDIR)/spellsuggest.obj:	$(OUTDIR) spellsuggest.c  $(INCL)
 
+$(OUTDIR)/strings.obj:	$(OUTDIR) strings.c  $(INCL)
+
 $(OUTDIR)/syntax.obj:	$(OUTDIR) syntax.c  $(INCL)
 
 $(OUTDIR)/tag.obj:	$(OUTDIR) tag.c  $(INCL)
@@ -2012,6 +2015,7 @@
 	proto/spell.pro \
 	proto/spellfile.pro \
 	proto/spellsuggest.pro \
+	proto/strings.pro \
 	proto/syntax.pro \
 	proto/tag.pro \
 	proto/term.pro \