Fix 'not in' as continuation operator in IEx

[AlexGx]

Current

iex> 4
iex> in [1, 2]
false

```elixir
iex> 4
iex> not in [1, 2]
** (SyntaxError) invalid syntax found on iex:4:1:
    error: syntax error before: {in_op,{4,1,nil},'not in',{4,5,nil}}
    │
  4 │ not in [1, 2]
    │ ^
    │
    └─ iex:4:1
    (iex 1.21.0-dev) lib/iex/server.ex:403: IEx.Server.__parse__/3
    (stdlib 8.0.2) io_lib.erl:1649: :io_lib.get_until/4
    (kernel 11.0.3) group.erl:407: :group.xterm/3
    (stdlib 8.0.2) gen_statem.erl:3741: :gen_statem.loop_state_callback/11
    (stdlib 8.0.2) proc_lib.erl:333: :proc_lib.init_p_do_apply/3

Expected

true, same as:

iex> 4 not in [1, 2]
true

Tokenizer represents composite not in as four-element token, like: {in_op,{4,1,nil},'not in',{4,5,nil}}

[josevalim]

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