patch 8.2.0601: Vim9: :unlet is not compiled

Problem:    Vim9: :unlet is not compiled.
Solution:   Implement :unlet instruction and check for errors.
diff --git a/src/version.c b/src/version.c
index d4a1f4f..21400bb 100644
--- a/src/version.c
+++ b/src/version.c
@@ -747,6 +747,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    601,
+/**/
     600,
 /**/
     599,