patch 8.2.3996: Vim9: type checking lacks information about declared type

Problem:    Vim9: type checking for list and dict lacks information about
            declared type.
Solution:   Add dv_decl_type and lv_decl_type.  Refactor the type stack to
            store two types in each entry.

src/testdir/test_vim9_assign.vim

@@ -639,6 +639,23 @@ def Test_extend_list()
       extend(test_null_list(), ['x'])
   END
   CheckScriptFailure(lines, 'E1134:', 2)
+
+  # using global var has no declared type
+  g:myList = []
+  g:myList->extend([1])
+  g:myList->extend(['x'])
+  assert_equal([1, 'x'], g:myList)
+  unlet g:myList
+
+  # using declared list gives an error
+  lines =<< trim END
+      var l: list<number>
+      g:myList = l
+      g:myList->extend([1])
+      g:myList->extend(['x'])
+  END
+  CheckDefExecAndScriptFailure(lines, 'E1013: Argument 2: type mismatch, expected list<number> but got list<string>', 4)
+  unlet g:myList
 enddef
 
 def Test_extend_dict()
@@ -963,6 +980,23 @@ def Test_assignment_dict()
   var anyDict: dict<any> = {a: 0}
   assert_equal({a: 0, b: 'x'}, extend(anyDict, {b: 'x'}))
 
+  # using global var, which has no declared type
+  g:myDict = {}
+  g:myDict->extend({a: 1})
+  g:myDict->extend({b: 'x'})
+  assert_equal({a: 1, b: 'x'}, g:myDict)
+  unlet g:myDict
+
+  # using list with declared type gives an error
+  lines =<< trim END
+      var d: dict<number>
+      g:myDict = d
+      g:myDict->extend({a: 1})
+      g:myDict->extend({b: 'x'})
+  END
+  CheckDefExecAndScriptFailure(lines, 'E1013: Argument 2: type mismatch, expected dict<number> but got dict<string>', 4)
+  unlet g:myDict
+
   # assignment to script-local dict
   lines =<< trim END
     vim9script