Fix autocomplete for many_to_many associations

[damonvjanis]

Using ElixirLS with VS Code, I noticed that schemas with a many_to_many relationship weren't getting autocompletions as I composed ecto queries.

The output was complaining about the %ManyToMany{} struct not having a :related_key, and I traced it back to here. I've confirmed that this change does fix the issue and allows you to get autocompletions for schemas with many_to_many relationships.

[lukaszsamson]

@damonvjanis can you add a test case?

[damonvjanis]

@lukaszsamson I got one added. I think the new fake schema is correct, but I'm not an expert on Ecto internals. The Ecto.Association.ManyToMany struct (https://github.com/elixir-ecto/ecto/blob/v3.5.6/lib/ecto/association.ex#L1003) doesn't define a related_key field which is what causes the issue I observed, and so when I added a many_to_many association in the fakes that way some tests started failing, but the suggested change I have fixes them. I added another test that is functionally identical to the one above it, but is labeled clearly to make the documentation more explicit.

[lukaszsamson]

Thanks