patch 8.2.1867: Vim9: argument to add() not checked for blob

Problem:    Vim9: argument to add() not checked for blob.
Solution:   Add the BLOBAPPEND instruction.
diff --git a/src/vim9.h b/src/vim9.h
index 4cece6d..68b9bfa 100644
--- a/src/vim9.h
+++ b/src/vim9.h
@@ -126,6 +126,7 @@
     ISN_ANYINDEX,   // [expr] runtime index
     ISN_ANYSLICE,   // [expr:expr] runtime slice
     ISN_SLICE,	    // drop isn_arg.number items from start of list
+    ISN_BLOBAPPEND, // append to a blob, like add()
     ISN_GETITEM,    // push list item, isn_arg.number is the index
     ISN_MEMBER,	    // dict[member]
     ISN_STRINGMEMBER, // dict.member using isn_arg.string