feat: Allow file to be a function

[FoxxMD]

Enables users to provide a function that dynamically changes the base log file name on roll

This PR supersedes #58

This solution is more elegant (IMO) as it does not add additional options.

---

Particularly useful if a user prefers timestamp-based file names rather than pure numbers IE

const rollingDest = await pinoRoll({
    file: () => `${path.resolve(logPath, 'app')}-${dayjs().format('YYYY-MM-DD')}`,
    size: 10,
    frequency: 'daily',
    extension: '.log',
    mkdir: true
});

So that file names look like:

app-2024-02-21.1.log
app-2024-02-22.1.log
app-2024-02-23.1.log
app-2024-02-23.2.log

[FoxxMD]

Hi @feugy is this on your radar at all? Please let me know if there's something blocking this.

[mcollina]

lgtm