patch 8.2.1257: Vim9: list unpack doesn't work at the script level

Problem:    Vim9: list unpack doesn't work at the script level.
Solution:   Detect unpack assignment better. (closes #6494)

src/ex_docmd.c

@@ -3282,11 +3282,20 @@ find_ex_command(
 	// after the "]" by to_name_const_end(): check if a "=" follows.
 	// If "[...]" has a line break "p" still points at the "[" and it can't
 	// be an assignment.
-	if (*eap->cmd == '[' && (p == eap->cmd || *skipwhite(p) != '='))
+	if (*eap->cmd == '[')
+	{
+	    p = to_name_const_end(eap->cmd);
+	    if (p == eap->cmd || *skipwhite(p) != '=')
 	    {
 		eap->cmdidx = CMD_eval;
 		return eap->cmd;
 	    }
+	    if (p > eap->cmd && *skipwhite(p) == '=')
+	    {
+		eap->cmdidx = CMD_let;
+		return eap->cmd;
+	    }
+	}
 
 	// Recognize an assignment if we recognize the variable name:
 	// "g:var = expr"

src/testdir/test_vim9_script.vim

@@ -169,6 +169,18 @@ def Test_assignment_list()
   let somelist = rand() > 0 ? [1, 2, 3] : ['a', 'b', 'c']
 enddef
 
+def Test_assignment_list_vim9script()
+  let lines =<< trim END
+    vim9script
+    let v1: number
+    let v2: number
+    let v3: number
+    [v1, v2, v3] = [1, 2, 3]
+    assert_equal([1, 2, 3], [v1, v2, v3])
+  END
+  call CheckScriptSuccess(lines)
+enddef
+
 def Test_assignment_dict()
   let dict1: dict<bool> = #{one: false, two: true}
   let dict2: dict<number> = #{one: 1, two: 2}

src/version.c

@@ -754,6 +754,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1257,
 /**/
     1256,
 /**/