introduce `accounts.email.order` for aerc accounts

[RyanGibb]

Description

Nix does not preserve attrset order, instead sorting attributes by alphabetical order https://github.com/NixOS/nixpkgs/issues/81986.

However, aerc displays accounts based on their order in .config/aerc/accounts.conf.

This PR introduces a accounts.email.order list that provides an optional order for accounts in accounts.email.accounts.

I implement support for this in the aerc module, and I suspect other front-ends might also be able to benefit from this.

If a partial order is provided those will be ordered after the primary account, and any unordered accounts will be included with the previous alphabetical behavior after.

Checklist

• [x] Change is backwards compatible.

  • The primary account is still displayed first and if no order list is provided the previous behavior is maintained.

• [x] Code formatted with ./format.

• [x] Code tested through nix-shell --pure tests -A run.all or nix develop --ignore-environment .#all using Flakes.

• [ ] Test cases updated/added. See example.

• [x] Commit messages are formatted like

  {component}: {description}
  
  {long description}
  

  See CONTRIBUTING for more information and recent commit messages for examples.

Maintainer CC

@lukasngl