Mark machine applicable fixes as preferred

This allows us to run the auto fix command from vscode to automatically
fix all diagnostics in the file.

They are also distinguished in the UI.

crates/rust-analyzer/src/lsp_ext.rs

@@ -243,6 +243,7 @@ impl Request for CodeActionRequest {
 }
 
 #[derive(Debug, PartialEq, Clone, Default, Deserialize, Serialize)]
+#[serde(rename_all = "camelCase")]
 pub struct CodeAction {
     pub title: String,
     #[serde(skip_serializing_if = "Option::is_none")]
@@ -256,6 +257,8 @@ pub struct CodeAction {
     // pub command: Option<lsp_types::Command>,
     #[serde(skip_serializing_if = "Option::is_none")]
     pub edit: Option<SnippetWorkspaceEdit>,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub is_preferred: Option<bool>,
 }
 
 #[derive(Debug, Eq, PartialEq, Clone, Default, Deserialize, Serialize)]