vim-patch:8.2.1756: Vim9: :let will soon be disallowed (#37063)

Problem:    Vim9: :let will soon be disallowed.
Solution:   Add v:disallow_let temporarily.  Fix tests.

cfcd011fcd

The change to use checkforcmd() is already included in the port of patch
9.0.1505. This commit adds the missing :const check.

N/A patches:
vim-patch:8.2.1397: Vim9: return type of maparg() not adjusted for fourth arg
vim-patch:8.2.1623: Vim9: using :call where it is not needed
vim-patch:8.2.1766: Vim9: Some tests are still using :let
vim-patch:8.2.1788: Vim9: still allows :let for declarations
vim-patch:8.2.1882: Vim9: v:disallow_let is no longer needed

Co-authored-by: Bram Moolenaar <Bram@vim.org>

test/old/testdir/test_map_functions.vim

@@ -116,6 +116,19 @@ func Test_maparg()
   unlet d
 endfunc
 
+" def Test_vim9_maparg()
+"   nmap { w
+"   var one: string = maparg('{')
+"   assert_equal('w', one)
+"   var two: string = maparg('{', 'n')
+"   assert_equal('w', two)
+"   var three: string = maparg('{', 'n', 0)
+"   assert_equal('w', three)
+"   var four: dict<any> = maparg('{', 'n', 0, 1)
+"   assert_equal(['{', 'w', 'n'], [four.lhs, four.rhs, four.mode])
+"   nunmap {
+" enddef
+
 func Test_mapcheck()
   call assert_equal('', mapcheck('a'))
   call assert_equal('', mapcheck('abc'))