patch 8.2.1756: Vim9: :let will soon be disallowed
Problem: Vim9: :let will soon be disallowed.
Solution: Add v:disallow_let temporarily. Fix tests.
diff --git a/src/vim.h b/src/vim.h
index 9da878e..c43ec83 100644
--- a/src/vim.h
+++ b/src/vim.h
@@ -1992,7 +1992,8 @@
#define VV_ECHOSPACE 93
#define VV_ARGV 94
#define VV_COLLATE 95
-#define VV_LEN 96 // number of v: vars
+#define VV_DISALLOW_LET 96 // TODO: remove again
+#define VV_LEN 97 // number of v: vars
// used for v_number in VAR_BOOL and VAR_SPECIAL
#define VVAL_FALSE 0L // VAR_BOOL