Define `Song` struct

A canonical `Song` struct should be defined that allows storage of song simfile data for all song file formats.

The struct will need to support header information (such as title, artist, etc.), alongside repeated chart data (steps, timing, etc.). This format will likely resemble the SSC format the closest, but will also allow storing data specific to other file formats. The structure will be able to be mapped both to and from each of the respective file formats.

Most of the fields will be optional, since not all of the fields will be provided with each respective format.

Header fields to be included:

- [x] Title (#13)
- [x] Subtitle (#17)
- [x] Artist (#18)
- [ ] Genre (#21)
- [ ] Origin (#22)
- [ ] Banner
- [x] Background
- [ ] Preview Video
- [ ] Jacket
- [ ] CD Image
- [ ] Disc Image
- [ ] Lyrics file path
- [ ] CD Title
- [x] Audio preview
- [x] Selectable (#23)
- [ ] Last Second Hint
- [ ] Menu color

Chart fields:

- [ ] Chart Name
- [ ] Description
- [ ] Type
- [x] Style
- [x] Difficulty
- [x] Meter
- [ ] Radar
- [x] Credit
- [x] Notes
- [ ] Key Sounds
- [ ] Attacks
- [ ] Tick Counts
- [ ] Combos
- [ ] BPMs
- [ ] Display BPMs
- [ ] Stops
- [ ] Delays
- [ ] Warps
- [x] Music file
- [ ] Music file MD5 hash
- [x] Offset
- [ ] Time Signatures
- [ ] Scrolls
- [ ] Scroll Speeds
- [ ] Fake Segments
- [ ] Labeled Segments
- [ ] Vivid offset