chore: Extract languages from zed crate (#8270)

- Moves languages module from `zed` into a separate crate. That way we
have less of a long pole at the end of compilation.
- Removes moot dependencies on editor/picker. This is totally harmless
and might help in the future if we decide to decouple picker from
editor.

Before:
```
Number of crates that depend on 'picker' but not on 'editor': 1
Total number of crates that depend on 'picker': 13
Total number of crates that depend on 'editor': 30
```
After:
```
Number of crates that depend on 'picker' but not on 'editor': 5
Total number of crates that depend on 'picker': 12
Total number of crates that depend on 'editor': 26
```
The more crates depend on just picker but not editor, the better in that
case.

Release Notes:

- N/A

crates/languages/src/dart.rs

@@ -0,0 +1,54 @@
+use anyhow::{anyhow, Result};
+use async_trait::async_trait;
+use language::{LanguageServerName, LspAdapter, LspAdapterDelegate};
+use lsp::LanguageServerBinary;
+use std::{any::Any, path::PathBuf};
+
+pub struct DartLanguageServer;
+
+#[async_trait]
+impl LspAdapter for DartLanguageServer {
+    fn name(&self) -> LanguageServerName {
+        LanguageServerName("dart".into())
+    }
+
+    fn short_name(&self) -> &'static str {
+        "dart"
+    }
+
+    async fn fetch_latest_server_version(
+        &self,
+        _: &dyn LspAdapterDelegate,
+    ) -> Result<Box<dyn 'static + Send + Any>> {
+        Ok(Box::new(()))
+    }
+
+    async fn fetch_server_binary(
+        &self,
+        _: Box<dyn 'static + Send + Any>,
+        _: PathBuf,
+        _: &dyn LspAdapterDelegate,
+    ) -> Result<LanguageServerBinary> {
+        Err(anyhow!("dart must me installed from dart.dev/get-dart"))
+    }
+
+    async fn cached_server_binary(
+        &self,
+        _: PathBuf,
+        _: &dyn LspAdapterDelegate,
+    ) -> Option<LanguageServerBinary> {
+        Some(LanguageServerBinary {
+            path: "dart".into(),
+            env: None,
+            arguments: vec!["language-server".into(), "--protocol=lsp".into()],
+        })
+    }
+
+    fn can_be_reinstalled(&self) -> bool {
+        false
+    }
+
+    async fn installation_test_binary(&self, _: PathBuf) -> Option<LanguageServerBinary> {
+        None
+    }
+}