brotli を 1.x から 2.x へアップグレード

### Changed
 - Use the nullish coalescing (??) operator to check if the value is defined (#38)
 - Use imports field to ship differential implementation for node and browser

### Changed

- **Breaking:** Converted the package to ESM only. All public entry points use
  named `compress` and/or `decompress` exports with no default export.
- **Breaking:** `compress()` now returns `Promise<Uint8Array | null>` and must
  be awaited. Compression options, output bytes, and `null` results are unchanged.
  `decompress()` remains synchronous.
- Public entry points are limited to the package root, `brotli/compress`, and
  `brotli/decompress`, including `.js` aliases for both standalone modules.
  Standalone modules export their named function.
- Encoder initialization is deferred until the first compression call. Concurrent
  calls share one initialization Promise, including failures, with no automatic
  retries. Initialization and runtime errors reject the compression Promise.
- Removed top-level await and enabled modern Node.js `require()` through every
  public entry point. Unflagged ESM loading is supported in Node 20.19+, 22.12+,
  and later releases; see the [Node documentation](https://nodejs.org/api/modules.html#loading-ecmascript-modules-using-require).
  The distribution remains ESM only.
- Encoding still runs on the calling thread without workers. Keep the input,
  options, and dictionary unchanged until compression settles; no input copying
  is added.

### Added

- TypeScript declarations for the package root and standalone modules, including
  their `.js` aliases and the `CompressOptions` type.
- Tests for ESM and CommonJS exports, lazy and shared initialization, Promise
  results and failures, and browser bundles, including dictionary initialization
  and decompression without the encoder.

### Migration

```javascript
import { compress, decompress } from 'brotli';

const compressed = await compress(input);
const original = decompress(compressed);
```

To load either function independently:

```javascript
import { compress } from 'brotli/compress';
import { decompress } from 'brotli/decompress';
```

Modern Node.js CommonJS consumers can destructure named exports from the root
or standalone modules:

```javascript
const { compress, decompress } = require('brotli');
// Alternatively: const { compress } = require('brotli/compress');

compress(input).then(compressed => {
  const original = decompress(compressed);
}).catch(console.error);
```

Older Node.js CommonJS consumers can continue to use dynamic `import()`.

brotli 1.3.3 と 2.1.0 の間で、Vigia は削除されたエクスポートを 0 件、変更されたシグネチャを 2 件、削除または変更されたクラス／インターフェースのメンバーを 0 件検出しました。

brotli 2.1.0 では 0 件のエクスポートまたはメンバーが新たに非推奨になりました。