patch 8.2.1447: Vim9: return type of keys() is list<any>

Problem:    Vim9: return type of keys() is list<any>.
Solution:   Should be list<string>. (closes #6711)

src/evalfunc.c

@@ -715,7 +715,7 @@ static funcentry_T global_functions[] =
     {"js_encode",	1, 1, FEARG_1,	  ret_string,	f_js_encode},
     {"json_decode",	1, 1, FEARG_1,	  ret_any,	f_json_decode},
     {"json_encode",	1, 1, FEARG_1,	  ret_string,	f_json_encode},
-    {"keys",		1, 1, FEARG_1,	  ret_list_any,	f_keys},
+    {"keys",		1, 1, FEARG_1,	  ret_list_string, f_keys},
     {"last_buffer_nr",	0, 0, 0,	  ret_number,	f_last_buffer_nr}, // obsolete
     {"len",		1, 1, FEARG_1,	  ret_number,	f_len},
     {"libcall",		3, 3, FEARG_3,	  ret_string,	f_libcall},

src/testdir/test_vim9_func.vim

@@ -1283,6 +1283,11 @@ def Test_insert_return_type()
   assert_equal(6, res)
 enddef
 
+def Test_keys_return_type()
+  const var: list<string> = #{a: 1, b: 2}->keys()
+  assert_equal(['a', 'b'], var)
+enddef
+
 def Test_reverse_return_type()
   let l = reverse([1, 2, 3])
   let res = 0

src/version.c

@@ -754,6 +754,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1447,
 /**/
     1446,
 /**/