Identify default mailbox

Currently, the mailbox list command returns a list of the following type of elements:

{
    "id": "[Gmail]/Starred",
    "name": "[Gmail]/Starred",
    "total": null,
    "unread": null
}

For normal CLI interactions this is good enough, but for a UI (even himalaya-tui) we need a default folder indicator. This is currently done by matching 'inbox' case-insensitively with the fetched mailboxes and the first one is choosen (Looks like the case in himalaya-tui as well: src/tui/update.rs).

Proposal: a default boolean field along with existing for mailbox list entries.

PS: I'm not sure about the RFC specifications whether they say that this is a fixed value or not. I'd like to know more about this too.