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/Filelist b/Filelist
index 579344b..8324ada 100644
--- a/Filelist
+++ b/Filelist
@@ -134,6 +134,7 @@
 		src/spell.h \
 		src/spellfile.c \
 		src/spellsuggest.c \
+		src/strings.c \
 		src/structs.h \
 		src/syntax.c \
 		src/tag.c \
@@ -296,6 +297,7 @@
 		src/proto/spell.pro \
 		src/proto/spellfile.pro \
 		src/proto/spellsuggest.pro \
+		src/proto/strings.pro \
 		src/proto/syntax.pro \
 		src/proto/tag.pro \
 		src/proto/term.pro \