feat: add storageRune.default and re-add constants codegen

[harrysolovay]

Closes #618

storageRune.default

import { System } from "polkadot_dev/mod.js"

assertEquals(
  await System.Account.default().run(),
  {
    nonce: 0,
    consumers: 0,
    providers: 0,
    sufficients: 0,
    data: { free: 0n, reserved: 0n, miscFrozen: 0n, feeFrozen: 0n }
  }
)

Constant codegen

import { System } from "polkadot_dev/mod.js"

System.SS58Prefix // `number`

[tjjfvi]

Given the direction we're going on the storage codegen I'm not sure we want constants codegen