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.