feat(editor): eslint/tsconfig migration + type fixes

@robonen/editor: migrate to eslint flat config + composite tsconfig; fix
convergence test type annotations.

vue/editor/src/commands/chain.ts

@@ -0,0 +1,16 @@
+import type { Command } from '../state';
+
+/**
+ * Combine commands into one that runs them in order and stops at the first that
+ * applies (returns `true`). The standard way to bind several fallbacks to a key.
+ */
+export function chainCommands(...commands: readonly Command[]): Command {
+  return (state, dispatch, view) => {
+    for (const command of commands) {
+      if (command(state, dispatch, view))
+        return true;
+    }
+
+    return false;
+  };
+}