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_.
diff --git a/src/version.c b/src/version.c
index 8dba7a6..5ac7d2e 100644
--- a/src/version.c
+++ b/src/version.c
@@ -751,6 +751,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    2108,
+/**/
     2107,
 /**/
     2106,