fileStorage: add file handles

This changes how files work. There is now an open function that
translates a path to a file handle (id). Then this handle is used
to perform other actions on the file. The file contents are moved
to a separate table so that the actual file storage is independent
of the database (e.g. in the future, we may use File Access API).

We store a hash of the file contents in the metadata file so we can
detect file changes without having to compare file contents.

We also separate the change and add actions.

src/utils/crypto.test.ts

@@ -0,0 +1,10 @@
+// SPDX-License-Identifier: MIT
+// Copyright (c) 2022 The Pybricks Authors
+
+import { sha256Digest } from './crypto';
+
+test('sha256Digest', async () => {
+    await expect(sha256Digest('test')).resolves.toBe(
+        '9f86d081884c7d659a2feaa0c55ad015a3bf4f1b2b0b822cd15d6c15b0f00a08',
+    );
+});

src/utils/crypto.ts

@@ -0,0 +1,11 @@
+// SPDX-License-Identifier: MIT
+// Copyright (c) 2022 The Pybricks Authors
+
+const encoder = new TextEncoder();
+
+export async function sha256Digest(data: string): Promise<string> {
+    const hash = await window.crypto.subtle.digest('SHA-256', encoder.encode(data));
+    return Array.from(new Uint8Array(hash))
+        .map((n) => n.toString(16).padStart(2, '0'))
+        .join('');
+}