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_vms.mms b/src/Make_vms.mms
index 4928b0b..3f65201 100644
--- a/src/Make_vms.mms
+++ b/src/Make_vms.mms
@@ -393,6 +393,7 @@
 	spell.c \
 	spellfile.c \
 	spellsuggest.c \
+	strings.c \
 	syntax.c \
 	tag.c \
 	term.c \
@@ -512,6 +513,7 @@
 	spell.obj \
 	spellfile.obj \
 	spellsuggest.obj \
+	strings.obj \
 	syntax.obj \
 	tag.obj \
 	term.obj \
@@ -1048,6 +1050,10 @@
  ascii.h keymap.h term.h macros.h option.h structs.h \
  regexp.h gui.h beval.h [.proto]gui_beval.pro alloc.h ex_cmds.h spell.h \
  proto.h errors.h globals.h
+strings.obj : strings.c vim.h [.auto]config.h feature.h os_unix.h \
+ ascii.h keymap.h term.h macros.h option.h structs.h \
+ regexp.h gui.h beval.h [.proto]gui_beval.pro alloc.h ex_cmds.h spell.h \
+ proto.h errors.h globals.h
 syntax.obj : syntax.c vim.h [.auto]config.h feature.h os_unix.h \
  ascii.h keymap.h term.h macros.h structs.h regexp.h \
  gui.h beval.h [.proto]gui_beval.pro option.h ex_cmds.h proto.h \