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_ami.mak b/src/Make_ami.mak
index 91246d9..678f999 100644
--- a/src/Make_ami.mak
+++ b/src/Make_ami.mak
@@ -162,6 +162,7 @@
 	spell.c \
 	spellfile.c \
 	spellsuggest.c \
+	strings.c \
 	syntax.c \
 	tag.c \
 	term.c \