patch 8.2.2878: Vim9: for loop list unpack only allows for one "_"

Problem:    Vim9: for loop list unpack only allows for one "_".
Solution:   Drop the value when the variable is "_". (closes #8232)

src/testdir/test_vim9_script.vim

@@ -2500,6 +2500,12 @@ def Test_for_loop_unpack()
       endfor
       assert_equal(['global', 'buf', 'win', 'tab', '1', '2', '3', '4'], slist)
       unlet! g:globalvar b:bufvar w:winvar t:tabvar
+
+      var res = []
+      for [_, n, _] in [[1, 2, 3], [4, 5, 6]]
+        res->add(n)
+      endfor
+      assert_equal([2, 5], res)
   END
   CheckDefAndScriptSuccess(lines)
 

src/version.c

@@ -750,6 +750,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    2878,
 /**/
     2877,
 /**/

src/vim9compile.c

@@ -7776,6 +7776,12 @@ compile_for(char_u *arg_start, cctx_T *cctx)
 						     0, 0, type, name) == FAIL)
 		goto failed;
 	}
+	else if (varlen == 1 && *arg == '_')
+	{
+	    // Assigning to "_": drop the value.
+	    if (generate_instr_drop(cctx, ISN_DROP, 1) == NULL)
+		goto failed;
+	}
 	else
 	{
 	    if (lookup_local(arg, varlen, NULL, cctx) == OK)