patch 8.2.0847: typval related code is spread out

Problem:    Typval related code is spread out.
Solution:   Move code to new typval.c file. (Yegappan Lakshmanan, closes #6093)
diff --git a/src/README.md b/src/README.md
index 624bd6a..6cc0704 100644
--- a/src/README.md
+++ b/src/README.md
@@ -84,6 +84,7 @@
 textobject.c	| text objects
 textprop.c	| text properties
 time.c		| time and timer functions
+typval.c	| vim script type/value functions
 undo.c		| undo and redo
 usercmd.c	| user defined commands
 userfunc.c	| user defined functions