This commit adds a method to make it easier to check that popen handle
is closed.

Closes tarantool#11492

@TarantoolBot document
Title: `popen_handle:is_closed()` method

The new `is_closed()` method returns `true` if the popen handle is
closed, and `false` otherwise.

Usage example:

```lua
local ph = popen.new(<...>)
assert(ph:is_closed() == false)
...
ph:close()
assert(ph:is_closed() == true)
```