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.

extensions/elixir/src/language_servers/lexical.rs

@@ -0,0 +1,130 @@
+use std::fs;
+
+use zed::lsp::{Completion, CompletionKind, Symbol};
+use zed::{CodeLabel, CodeLabelSpan, LanguageServerId};
+use zed_extension_api::{self as zed, Result};
+
+pub struct Lexical {
+    cached_binary_path: Option<String>,
+}
+
+impl Lexical {
+    pub const LANGUAGE_SERVER_ID: &'static str = "lexical";
+
+    pub fn new() -> Self {
+        Self {
+            cached_binary_path: None,
+        }
+    }
+
+    pub fn language_server_binary_path(
+        &mut self,
+        language_server_id: &LanguageServerId,
+        _worktree: &zed::Worktree,
+    ) -> Result<String> {
+        if let Some(path) = &self.cached_binary_path {
+            if fs::metadata(path).map_or(false, |stat| stat.is_file()) {
+                return Ok(path.clone());
+            }
+        }
+
+        zed::set_language_server_installation_status(
+            &language_server_id,
+            &zed::LanguageServerInstallationStatus::CheckingForUpdate,
+        );
+        let release = zed::latest_github_release(
+            "lexical-lsp/lexical",
+            zed::GithubReleaseOptions {
+                require_assets: true,
+                pre_release: false,
+            },
+        )?;
+
+        let asset_name = format!("lexical-{version}.zip", version = release.version);
+
+        let asset = release
+            .assets
+            .iter()
+            .find(|asset| asset.name == asset_name)
+            .ok_or_else(|| format!("no asset found matching {:?}", asset_name))?;
+
+        let version_dir = format!("lexical-{}", release.version);
+        let binary_path = format!("{version_dir}/lexical/bin/start_lexical.sh");
+
+        if !fs::metadata(&binary_path).map_or(false, |stat| stat.is_file()) {
+            zed::set_language_server_installation_status(
+                &language_server_id,
+                &zed::LanguageServerInstallationStatus::Downloading,
+            );
+
+            zed::download_file(
+                &asset.download_url,
+                &version_dir,
+                zed::DownloadedFileType::Zip,
+            )
+            .map_err(|e| format!("failed to download file: {e}"))?;
+
+            let entries =
+                fs::read_dir(".").map_err(|e| format!("failed to list working directory {e}"))?;
+            for entry in entries {
+                let entry = entry.map_err(|e| format!("failed to load directory entry {e}"))?;
+                if entry.file_name().to_str() != Some(&version_dir) {
+                    fs::remove_dir_all(&entry.path()).ok();
+                }
+            }
+        }
+
+        self.cached_binary_path = Some(binary_path.clone());
+        Ok(binary_path)
+    }
+
+    pub fn label_for_completion(&self, completion: Completion) -> Option<CodeLabel> {
+        match completion.kind? {
+            CompletionKind::Module
+            | CompletionKind::Class
+            | CompletionKind::Interface
+            | CompletionKind::Struct => {
+                let name = completion.label;
+                let defmodule = "defmodule ";
+                let code = format!("{defmodule}{name}");
+
+                Some(CodeLabel {
+                    code,
+                    spans: vec![CodeLabelSpan::code_range(
+                        defmodule.len()..defmodule.len() + name.len(),
+                    )],
+                    filter_range: (0..name.len()).into(),
+                })
+            }
+            CompletionKind::Function | CompletionKind::Constant => {
+                let name = completion.label;
+                let def = "def ";
+                let code = format!("{def}{name}");
+
+                Some(CodeLabel {
+                    code,
+                    spans: vec![CodeLabelSpan::code_range(def.len()..def.len() + name.len())],
+                    filter_range: (0..name.len()).into(),
+                })
+            }
+            CompletionKind::Operator => {
+                let name = completion.label;
+                let def_a = "def a ";
+                let code = format!("{def_a}{name} b");
+
+                Some(CodeLabel {
+                    code,
+                    spans: vec![CodeLabelSpan::code_range(
+                        def_a.len()..def_a.len() + name.len(),
+                    )],
+                    filter_range: (0..name.len()).into(),
+                })
+            }
+            _ => None,
+        }
+    }
+
+    pub fn label_for_symbol(&self, _symbol: Symbol) -> Option<CodeLabel> {
+        None
+    }
+}