Add MMKV interface (#5169)

src/storage/README.md

@@ -0,0 +1,62 @@
+# `#/storage`
+
+## Usage
+
+Import the correctly scoped store from `#/storage`. Each instance of `Storage`
+(the base class, not to be used directly), has the following interface:
+
+- `set([...scope, key], value)`
+- `get([...scope, key])`
+- `remove([...scope, key])`
+- `removeMany([...scope], [...keys])`
+
+For example, using our `device` store looks like this, since it's scoped to the
+device (the most base level scope):
+
+```typescript
+import { device } from '#/storage';
+
+device.set(['foobar'], true);
+device.get(['foobar']);
+device.remove(['foobar']);
+device.removeMany([], ['foobar']);
+```
+
+## TypeScript
+
+Stores are strongly typed, and when setting a given value, it will need to
+conform to the schemas defined in `#/storage/schema`. When getting a value, it
+will be returned to you as the type defined in its schema.
+
+## Scoped Stores
+
+Some stores are (or might be) scoped to an account or other identifier. In this
+case, storage instances are created with type-guards, like this:
+
+```typescript
+type AccountSchema = {
+  language: `${string}-${string}`;
+};
+
+type DID = `did:${string}`;
+
+const account = new Storage<
+  [DID],
+  AccountSchema
+>({
+  id: 'account',
+});
+
+account.set(
+  ['did:plc:abc', 'language'],
+  'en-US',
+);
+
+const language = account.get([
+  'did:plc:abc',
+  'language',
+]);
+```
+
+Here, if `['did:plc:abc']` is not supplied along with the key of
+`language`, the `get` will return undefined (and TS will yell at you).