implement `core/wallet-store-ext`

this is the core piece that provides persistence for the browser extension.

This storage provider should also implement encrypted-at-rest storage, see:

Encrypted Storage Given the extension will store private keys, we will encrypt the values at rest and decrypt into memory when the extension is unlocked. The general process would look like:

1. Derive a symmetric AES-GCM encryption key from the user’s extension password, using PBKDF
2. Including a random salt value is recommended.
3. To write data: serialize the data into a string, encrypt it with the key, store the ciphertext + IV + salt.
4. To read data: load the ciphertext + IV, decrypt with the PBKDF2 derived key, and parse into JSON object

The password is not stored anywhere.