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.

crates/languages/Cargo.toml

@@ -26,12 +26,9 @@ project.workspace = true
 regex.workspace = true
 rope.workspace = true
 rust-embed = "8.2.0"
-schemars.workspace = true
 serde.workspace = true
-serde_derive.workspace = true
 serde_json.workspace = true
 settings.workspace = true
-shellexpand.workspace = true
 smol.workspace = true
 task.workspace = true
 toml.workspace = true
@@ -39,12 +36,10 @@ tree-sitter-bash.workspace = true
 tree-sitter-c.workspace = true
 tree-sitter-cpp.workspace = true
 tree-sitter-css.workspace = true
-tree-sitter-elixir.workspace = true
 tree-sitter-embedded-template.workspace = true
 tree-sitter-go.workspace = true
 tree-sitter-gomod.workspace = true
 tree-sitter-gowork.workspace = true
-tree-sitter-heex.workspace = true
 tree-sitter-jsdoc.workspace = true
 tree-sitter-json.workspace = true
 tree-sitter-markdown.workspace = true