Support `ssc` file format

[Anders429]

Song structs should be able to be read from and written to the ssc format.

The ssc format is the standard song format for Stepmania 5, and therefore has high priority.

Format documentation is available here: https://github.com/stepmania/stepmania/wiki/ssc

Header tags:

• [ ] #VERSION
• [ ] #TITLE
• [ ] #SUBTITLE
• [ ] #ARTIST
• [ ] #TITLETRANSLIT
• [ ] #SUBTITLETRANSLIT
• [ ] #ARTISTTRANSLIT
• [ ] #GENRE
• [ ] #ORIGIN
• [ ] #CREDIT
• [ ] #BANNER
• [ ] #BACKGROUND
• [ ] #PREVIEWVID
• [ ] #JACKET
• [ ] #CDIMAGE
• [ ] #DISCIMAGE
• [ ] #LYRICSPATH
• [ ] #CDTITLE
• [ ] #MUSIC
• [ ] #PREVIEW
• [ ] #OFFSET
• [ ] #SAMPLESTART
• [ ] #SAMPLELENGTH
• [ ] #SELECTABLE
• [ ] #BPMS
• [ ] #DISPLAYBPM
• [ ] #STOPS
• [ ] #DELAYS
• [ ] #WARPS
• [ ] #TIMESIGNATURES
• [ ] #TICKCOUNTS
• [ ] #COMBOS
• [ ] #SPEEDS
• [ ] #SCROLLS
• [ ] #FAKES
• [ ] #LABELS
• [ ] #LASTSECONDHINT
• [ ] #BGCHANGES
• [ ] #KEYSOUNDS
• [ ] #ATTACKS

Chart tags:

• [ ] #NOTEDATA
• [ ] #CHARTNAME
• [ ] #STEPSTYPE
• [ ] #DESCRIPTION
• [ ] #CHARTSTYLE
• [ ] #DIFFICULTY
• [ ] #METER
• [ ] #RADARVALUES
• [ ] #CREDIT
• [ ] #NOTES
• [ ] #NOTES2

Cache-only header tags:

• [ ] #FIRSTSECOND
• [ ] #LASTSECOND
• [ ] #SONGFILENAME
• [ ] #HASMUSIC
• [ ] #HASBANNER

Deprecated header cache tags:

• [ ] #LASTBEATHINT
• [ ] #MUSICBYTES
• [ ] #FIRSTBEAT
• [ ] #LASTBEAT