patch 8.2.0601: Vim9: :unlet is not compiled

Problem:    Vim9: :unlet is not compiled.
Solution:   Implement :unlet instruction and check for errors.
diff --git a/src/vim9execute.c b/src/vim9execute.c
index b0e35b6..e675869 100644
--- a/src/vim9execute.c
+++ b/src/vim9execute.c
@@ -1068,6 +1068,12 @@
 		}
 		break;
 
+	    case ISN_UNLET:
+		if (do_unlet(iptr->isn_arg.unlet.ul_name,
+				       iptr->isn_arg.unlet.ul_forceit) == FAIL)
+		    goto failed;
+		break;
+
 	    // create a list from items on the stack; uses a single allocation
 	    // for the list header and the items
 	    case ISN_NEWLIST:
@@ -2108,6 +2114,11 @@
 	    case ISN_PUSHEXC:
 		smsg("%4d PUSH v:exception", current);
 		break;
+	    case ISN_UNLET:
+		smsg("%4d UNLET%s %s", current,
+			iptr->isn_arg.unlet.ul_forceit ? "!" : "",
+			iptr->isn_arg.unlet.ul_name);
+		break;
 	    case ISN_NEWLIST:
 		smsg("%4d NEWLIST size %lld", current,
 					    (long long)(iptr->isn_arg.number));