This commit introduces support for 'sets', a feature that simplifies
referencing multiple relays or naddrs with a single word.
Sets are stored in the config directory under `n34` as `config.toml` in
`TOML` format. The config path follows the `$XDG_CONFIG_HOME` standard,
defaulting to `$HOME/.config` if unset.
The `n34 sets` command allows managing sets, adding, removing, updating,
or displaying them. When using the main `--relays` flag, a set name can
be provided to automatically expand its relays (an error occurs if the
set contains no relays). Similarly, any command accepting naddrs can
reference a set to extract its naddrs.
Sets can also be merged by updating a set while specifying another
set in `--repo` merges their naddrs, while `--set-relays` merges their
relays. Removal follows the same pattern.
Signed-off-by: Awiteb <a@4rs.nl>
