patch 8.2.2303: Vim9: backtick expansion doesn't work for :foldopen

Problem:    Vim9: backtick expansion doesn't work for :foldopen.
Solution:   Do recognize backtick expansion. (closes #7621)
diff --git a/src/vim9compile.c b/src/vim9compile.c
index cf5cff8..67307f8 100644
--- a/src/vim9compile.c
+++ b/src/vim9compile.c
@@ -7505,6 +7505,13 @@
 	}
     }
 
+    if (eap->cmdidx == CMD_folddoopen || eap->cmdidx == CMD_folddoclosed)
+    {
+	// TODO: should only expand when appropriate for the command
+	eap->arg = skiptowhite(eap->arg);
+	has_expr = TRUE;
+    }
+
     if (has_expr && (p = (char_u *)strstr((char *)eap->arg, "`=")) != NULL)
     {
 	int	count = 0;