patch 8.2.2753: Vim9: cannot ignore an item in assignment unpack

Problem:    Vim9: cannot ignore an item in assignment unpack.
Solution:   Allow using an underscore.
diff --git a/src/vim9compile.c b/src/vim9compile.c
index 02c75c4..15d2c30e 100644
--- a/src/vim9compile.c
+++ b/src/vim9compile.c
@@ -6369,6 +6369,17 @@
     {
 	int		instr_count = -1;
 
+	if (var_start[0] == '_' && !eval_isnamec(var_start[1]))
+	{
+	    // Ignore underscore in "[a, _, b] = list".
+	    if (var_count > 0)
+	    {
+		var_start = skipwhite(var_start + 2);
+		continue;
+	    }
+	    emsg(_(e_cannot_use_underscore_here));
+	    goto theend;
+	}
 	vim_free(lhs.lhs_name);
 
 	/*
@@ -6388,11 +6399,6 @@
 	    semsg(_(e_cannot_assign_to_constant), lhs.lhs_name);
 	    goto theend;
 	}
-	if (is_decl && lhs.lhs_name[0] == '_' && lhs.lhs_name[1] == NUL)
-	{
-	    emsg(_(e_cannot_use_underscore_here));
-	    goto theend;
-	}
 
 	if (!heredoc)
 	{