chore: add ESLint with react-hooks

eslint.config.mjs

@@ -0,0 +1,45 @@
+import js from '@eslint/js';
+import prettier from 'eslint-config-prettier';
+import reactHooks from 'eslint-plugin-react-hooks';
+import globals from 'globals';
+import tseslint from 'typescript-eslint';
+
+export default tseslint.config(
+  { ignores: ['dist', 'coverage', 'demo/dist', 'demo/node_modules'] },
+
+  js.configs.recommended,
+  tseslint.configs.recommended,
+  reactHooks.configs.flat.recommended,
+
+  {
+    languageOptions: {
+      globals: { ...globals.browser, ...globals.es2021 },
+    },
+    rules: {
+      // CONTRIBUTING: no console.log / console.debug.
+      'no-console': ['error', { allow: ['info', 'warn', 'error'] }],
+      // The component's effect deps are hand-tuned on purpose, so this must
+      // be loud enough that omitting one needs a written justification.
+      'react-hooks/exhaustive-deps': 'error',
+      // `const { a, b, ...rest } = obj` is the idiomatic way to omit keys;
+      // the named bindings are the point, not dead code.
+      '@typescript-eslint/no-unused-vars': [
+        'error',
+        { ignoreRestSiblings: true, argsIgnorePattern: '^_' },
+      ],
+    },
+  },
+
+  {
+    files: ['test/**'],
+    languageOptions: { globals: { ...globals.node, ...globals.vitest } },
+  },
+
+  {
+    files: ['*.config.{js,ts}', 'demo/vite.config.ts'],
+    languageOptions: { globals: globals.node },
+  },
+
+  // Must stay last: turns off every rule Prettier already owns.
+  prettier
+);