patch 8.2.2108: Vim9: no test to check for :let error

Problem:    Vim9: no test to check for :let error.
Solution:   Add a test.  Rename tests from _let_ to _var_.

src/testdir/test_vim9_assign.vim

@@ -995,7 +995,7 @@ def Test_heredoc()
   delfunc! g:Func
 enddef
 
-def Test_let_func_call()
+def Test_var_func_call()
   var lines =<< trim END
     vim9script
     func GetValue()
@@ -1019,7 +1019,7 @@ def Test_let_func_call()
   delete('Xfinished')
 enddef
 
-def Test_let_missing_type()
+def Test_var_missing_type()
   var lines =<< trim END
     vim9script
     var name = g:unknown
@@ -1034,7 +1034,7 @@ def Test_let_missing_type()
   CheckScriptSuccess(lines)
 enddef
 
-def Test_let_declaration()
+def Test_var_declaration()
   var lines =<< trim END
     vim9script
     var name: string
@@ -1069,7 +1069,7 @@ def Test_let_declaration()
   unlet g:other_var
 enddef
 
-def Test_let_declaration_fails()
+def Test_var_declaration_fails()
   var lines =<< trim END
     vim9script
     final var: string
@@ -1089,7 +1089,7 @@ def Test_let_declaration_fails()
   CheckScriptFailure(lines, 'E475:')
 enddef
 
-def Test_let_type_check()
+def Test_var_type_check()
   var lines =<< trim END
     vim9script
     var name: string
@@ -1126,7 +1126,7 @@ enddef
 
 let g:dict_number = #{one: 1, two: 2}
 
-def Test_let_list_dict_type()
+def Test_var_list_dict_type()
   var ll: list<number>
   ll = [1, 2, 2, 3, 3, 3]->uniq()
   ll->assert_equal([1, 2, 3])
@@ -1142,6 +1142,10 @@ def Test_let_list_dict_type()
   CheckDefExecFailure(lines, 'E1012: Type mismatch; expected list<number> but got list<string>')
 enddef
 
+def Test_cannot_use_let()
+  CheckDefAndScriptFailure(['let a = 34'], 'E1126:', 1)
+enddef
+
 def Test_unlet()
   g:somevar = 'yes'
   assert_true(exists('g:somevar'))

src/version.c

@@ -750,6 +750,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    2108,
 /**/
     2107,
 /**/