Support map fields completion

[scottming]

fix this:

17:28:51.342 [info] Completion for %Lexical.Document.Position{line: 9, character: 7}
17:28:52.711 [error] ** (ErlangError) Erlang error: {:exception, :function_clause, [{Lexical.RemoteControl.Completion.Candidate, :from_elixir_sense, [%{call?: true, name: "a", origin: nil, subtype: :map_key, type: :field, type_spec: nil}], [file: 'lib/lexical/remote_control/completion/candidate.ex', line: 161]}, {Enum, :"-map/2-lists^map/1-0-", 2, [file: 'lib/enum.ex', line: 1658]}, {Lexical.RemoteControl.Completion, :elixir_sense_expand, 2, []}]}
    (kernel 8.5.3) erpc.erl:702: :erpc.call/5
    (server 0.1.0) lib/lexical/server/code_intelligence/completion.ex:68: Lexical.Server.CodeIntelligence.Completion.completions/3
    (server 0.1.0) lib/lexical/server/code_intelligence/completion.ex:37: Lexical.Server.CodeIntelligence.Completion.complete/4
    (server 0.1.0) lib/lexical/server/provider/handlers/completion.ex:12: Lexical.Server.Provider.Handlers.Completion.handle/2
    (server 0.1.0) lib/lexical/server/provider/queue.ex:99: anonymous fn/2 in Lexical.Server.Provider.Queue.State.as_task/2
    (elixir 1.14.3) lib/task/supervised.ex:89: Task.Supervised.invoke_mfa/2
    (elixir 1.14.3) lib/task/supervised.ex:34: Task.Supervised.reply/4
    (stdlib 4.2) proc_lib.erl:240: :proc_lib.init_p_do_apply/3