Merges multiple ics files into one stream

Try `cargo run -- ics-debug --tz America/New_York sample-data/nascar.ics sample-data/formula-1.ics`

Expected output is the next month or so of NASCAR and Formula 1 racing:

```
2025-08-03 00:00:00 EDT - NASCAR Cup - Iowa Corn 350
2025-08-03 00:00:00 EDT - F1 - Lenovo Hungarian Grand Prix
2025-08-10 00:00:00 EDT - NASCAR Cup - Go Bowling At The Glen (Watkins Glen)
2025-08-16 00:00:00 EDT - NASCAR Cup - Cook Out 400 (Richmond)
2025-08-23 00:00:00 EDT - NASCAR Cup - Coke Zero Sugar 400 (Daytona)
2025-08-31 00:00:00 EDT - NASCAR Cup -Playoff- Southern 500 (Darlington)
2025-08-31 00:00:00 EDT - F1 - Heineken Dutch Grand Prix
2025-09-07 00:00:00 EDT - NASCAR Cup -Playoff- Enjoy Illinois 300 (Gateway)
2025-09-07 00:00:00 EDT - F1 - Gran Premio d'Italia
2025-09-13 00:00:00 EDT - NASCAR Cup -Playoff- Bass Pro Shops Night Race (Bristol)
2025-09-21 00:00:00 EDT - F1 - Qatar Airways Azerbaijan Grand Prix
```

If the sample data is old, try the iCal links from <https://toomuchracing.com/calendar/>, e.g. `curl https://calendar.google.com/calendar/ical/fa9bjl6tu13dd10b066stoo5do%40group.calendar.google.com/public/basic.ics > sample-data/formula-1.ics`