patch 8.2.0149: maintaining a Vim9 branch separately is more work

Problem:    Maintaining a Vim9 branch separately is more work.
Solution:   Merge the Vim9 script changes.
diff --git a/src/proto.h b/src/proto.h
index e8af1e2..7836c52 100644
--- a/src/proto.h
+++ b/src/proto.h
@@ -226,6 +226,11 @@
 # include "usercmd.pro"
 # include "userfunc.pro"
 # include "version.pro"
+# ifdef FEAT_EVAL
+#  include "vim9compile.pro"
+#  include "vim9execute.pro"
+#  include "vim9script.pro"
+# endif
 # include "window.pro"
 
 # ifdef FEAT_LUA