fix(input): discard following keys when discarding <Cmd>/K_LUA (#36498)

Technically the current behavior does match documentation. However, the
keys following <Cmd>/K_LUA aren't normally received by vim.on_key()
callbacks either, so it does makes sense to discard them along with the
preceding key.

One may also argue that vim.on_key() callbacks should instead receive
the following keys together with the <Cmd>/K_LUA, but doing that may
cause some performance problems, and even in that case the keys should
still be discarded together.

runtime/lua/vim/_editor.lua

@@ -604,7 +604,8 @@ local on_key_cbs = {} --- @type table<integer,[function, table]>
 ---          are applied, and {typed} is the key(s) before mappings are applied.
 ---          {typed} may be empty if {key} is produced by non-typed key(s) or by the
 ---          same typed key(s) that produced a previous {key}.
----          If {fn} returns an empty string, {key} is discarded/ignored.
+---          If {fn} returns an empty string, {key} is discarded/ignored, and if {key}
+---          is [<Cmd>] then the "[<Cmd>]…[<CR>]" sequence is discarded as a whole.
 ---          When {fn} is `nil`, the callback associated with namespace {ns_id} is removed.
 ---@param ns_id integer? Namespace ID. If nil or 0, generates and returns a
 ---                      new |nvim_create_namespace()| id.