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

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

vue/editor/src/schema/attr-spec.ts

@@ -0,0 +1,11 @@
+import type { AttrValue } from '../model';
+
+/** Specification for a single attribute: default, requiredness, validation. */
+export interface AttrSpec<V extends AttrValue = AttrValue> {
+  readonly default?: V;
+  readonly required?: boolean;
+  readonly validate?: (value: unknown) => boolean;
+}
+
+/** Map of attribute name → {@link AttrSpec}. */
+export type AttrsSpec = Readonly<Record<string, AttrSpec>>;