feat(todo): edit the file right in the TODO preview (#1129)

The TODO preview is an editor on the file, as in IntelliJ: a CodeMirror view attached to the real document, so edits share the buffer with editor tabs and Save All saves them; no editing switch. The caret lands on the selected item, also when another item of the same file is selected. Read-only files fall back to the read-only preview. Dirty buffers are held rather than released (releasing would auto-save or drop the edits), and previewText reads the held text.

Co-authored-by: qtvbwfn <[email protected]>

src/editor/api.ts

@@ -590,6 +590,8 @@ export interface DocumentAttachment {
   doc: number;
   /** Current document text; build the view's state from it. */
   text(): string;
+  /** The document's language mode name (backend-assigned, as the editor tabs use). */
+  uiMode(): string | null;
   readOnly: boolean;
   /** Add to the view (one view at a time). */
   extension: Extension;