Guard struct completion against non-charlist cursor contexts

[mariari]

Code completion would fail on stuff like %MODULE

with an error like

23:27:24.599 [error] #PID<0.264764.0> running GtBridge.Http.Router (connection #PID<0.264761.0>, stream id 1) terminated
Server: localhost:1212 (http)
Request: POST /COMPLETE
** (exit) exited in: GenServer.call(#PID<0.264748.0>, {:complete, "%__MODULE__", "@doc \"\"\"\nFork a command log\n\"\"\"\n@spec fork(non_neg_integer() | :tip, t()) :: t()\ndef fork(at \\\\ :tip, from = %__MODULE__{id: id} \\\\ head()) do\n  unless from == :main or from in list() do\n    raise ArgumentError, \"cannot fork from unknown branch \#{inspect(from)}\"\n  end\n\n  branch = %__MODULE__:\"fork_\#{System.unique_integer([:positive])}\"\n  AL.Command.create_tables(branch)\n  AL.Command.copy_prefix(from, branch, at_time(at))\n  AL.Object.create_tables(branch)\n  AL.Object.hydrate_since(0, branch)\n  register(branch, from)\n  AL.Scheduler.start(branch)\n  branch\n%__MODULE__"}, :infinity)
    ** (EXIT) an exception was raised:
        ** (FunctionClauseError) no function clause matching in List.to_string/1
            (elixir 1.18.4) lib/list.ex:1119: List.to_string({:local_or_var, ~c"__MODULE__"})
            (gt_bridge 0.17.3) lib/gt_bridge/completion.ex:47: GtBridge.Completion.complete/4
            (gt_bridge 0.17.3) lib/gt_bridge/eval.ex:147: GtBridge.Eval.handle_call/3
            (stdlib 6.1) gen_server.erl:2381: :gen_server.try_handle_call/4
            (stdlib 6.1) gen_server.erl:2410: :gen_server.handle_msg/6
            (stdlib 6.1) proc_lib.erl:329: :proc_lib.init_p_do_apply/3

this patch seeks to rectify this issue