patch 8.1.1195: Vim script debugger functionality needs cleanup

Problem:    Vim script debugger functionality needs cleanup.
Solution:   Move debugger code to a separate file.  Add more tests. (Yegappan
            Lakshmanan, closes #4285)
diff --git a/src/proto.h b/src/proto.h
index 8b299ec..448233a 100644
--- a/src/proto.h
+++ b/src/proto.h
@@ -68,6 +68,7 @@
 # ifdef FEAT_CSCOPE
 #  include "if_cscope.pro"
 # endif
+# include "debugger.pro"
 # include "dict.pro"
 # include "diff.pro"
 # include "digraph.pro"