Use buku database directly

[flexibeast]

As of Emacs 29.1, Emacs can be built with built-in support for accessing SQLite databases (the "SQLITE3" feature).

Ebuku accessing the buku database directly would avoid:

• relying on buku's user interface;
• having to create a buku process;
• needing to parse the output of buku

and substantially simplify Ebuku's code.

However, Emacs 29.1 was only released in July 2023, and at this point i want to support releases back to 27.1. (The code currently has Package-Requires: ((emacs "25.1")), but that line hasn't been updated for a while, and 27.1 now seems like a reasonable minimum.)

It would be good if (say) ebuku-use-sqlite was an optional feature that could be enabled (perhaps even by default if emacs-version ≧ 29.1 and "SQLITE3" ∈ system-configuration-features).

This would be developed on a feature branch that would eventually get merged into the main branch.