Extract Elixir extension (#10948)

This PR extracts Elixir support into an extension and removes the
built-in Elixir support from Zed.

As part of this, [Lexical](https://github.com/lexical-lsp/lexical) has
been added as an available language server for Elixir.

Since the Elixir extension provides three different language servers,
you'll need to use the `language_servers` setting to select the one you
want to use:

#### Elixir LS

```json
{
  "languages": {
    "Elixir": {
      "language_servers": [ "elixir-ls", "!next-ls", "!lexical", "..."]
    }
  }
}
```

#### Next LS

```json
{
  "languages": {
    "Elixir": {
      "language_servers": [ "next-ls", "!elixir-ls", "!lexical", "..."]
    }
  }
}
```

#### Lexical

```json
{
  "languages": {
    "Elixir": {
      "language_servers": [ "lexical", "!elixir-ls", "!next-ls", "..."]
    }
  }
}
```

These can either go in your user settings or your project settings.

Release Notes:

- Removed built-in support for Elixir, in favor of making it available
as an extension.