Split `$kernel::account` into sub modules

We should refactor `$kernel::account` module into sub modules.

> this file has become pretty huge - we should probably split this up into multiple files - e.g.,:
>
> - `asm/kernel/transaction/lib/account/mod.masm`
> - `asm/kernel/transaction/lib/account/code.masm`
> - `asm/kernel/transaction/lib/account/storage.masm`
> - `asm/kernel/transaction/lib/account/vault.masm`
> - maybe something else?
>
_Originally posted by @bobbinth in https://github.com/0xMiden/miden-base/pull/2025#discussion_r2596181154_

All storage related procedures can go into `storage.masm`, vault-related ones in `vault.masm` and code-related ones in `code.masm`. The other procedures working with account and storage commitment, nonce, data loading during prologue can probably stay in `mod.masm`.

This makes sense to do after #2025 at the earliest, though even better after the account procedure info follow-up was done, since this will again touch a bunch of account internals.