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/proto.h b/src/proto.h
index f8aabc2..7a5cc41 100644
--- a/src/proto.h
+++ b/src/proto.h
@@ -226,6 +226,7 @@
 # include "textobject.pro"
 # include "textformat.pro"
 # include "time.pro"
+# include "typval.pro"
 # include "ui.pro"
 # include "undo.pro"
 # include "usercmd.pro"