patch 8.2.1788: Vim9: still allows :let for declarations

Problem:    Vim9: still allows :let for declarations.
Solution:   Make the default for v:disallow_let one.  It can still be set to
            zero to allow for using :let.
diff --git a/src/evalvars.c b/src/evalvars.c
index 319717e..fdc6f45 100644
--- a/src/evalvars.c
+++ b/src/evalvars.c
@@ -215,8 +215,8 @@
 	    // add to compat scope dict
 	    hash_add(&compat_hashtab, p->vv_di.di_key);
     }
-    vimvars[VV_VERSION].vv_nr = VIM_VERSION_100;
-    vimvars[VV_VERSIONLONG].vv_nr = VIM_VERSION_100 * 10000 + highest_patch();
+    set_vim_var_nr(VV_VERSION, VIM_VERSION_100);
+    set_vim_var_nr(VV_VERSIONLONG, VIM_VERSION_100 * 10000 + highest_patch());
 
     set_vim_var_nr(VV_SEARCHFORWARD, 1L);
     set_vim_var_nr(VV_HLSEARCH, 1L);
@@ -244,6 +244,9 @@
 
     set_vim_var_nr(VV_ECHOSPACE,    sc_col - 1);
 
+    // TODO: remove later
+    set_vim_var_nr(VV_DISALLOW_LET, 1);
+
     // Default for v:register is not 0 but '"'.  This is adjusted once the
     // clipboard has been setup by calling reset_reg_var().
     set_reg_var(0);
diff --git a/src/testdir/runtest.vim b/src/testdir/runtest.vim
index 08b1f8e..6b09b97 100644
--- a/src/testdir/runtest.vim
+++ b/src/testdir/runtest.vim
@@ -360,9 +360,6 @@
   qall!
 endfunc
 
-" TODO: remove later
-let v:disallow_let = 1
-
 " Source the test script.  First grab the file name, in case the script
 " navigates away.  g:testname can be used by the tests.
 let g:testname = expand('%')
diff --git a/src/version.c b/src/version.c
index 877d63d..ff1a97a 100644
--- a/src/version.c
+++ b/src/version.c
@@ -751,6 +751,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    1788,
+/**/
     1787,
 /**/
     1786,