Support `msd` file format

[Anders429]

The msd file format is a Song file format that should be supported.

It has been referred to as the "granddaddy" of many simfile formats, including dwi, sm, ssc, and many other derivatives. Its tag format is common among all of these formats. It is basically a subset of the other formats with a few differences, so it shouldn't be that much more work to support it as well.

A specification of the msd format can be found here (version 2.0β1): https://web.archive.org/web/20070606025338/http://doremi.kalin.to/ddr/msd_format.html

Tags:

• [ ] #FILE
• [x] #TITLE
• [x] #ARTIST
• [ ] #BPM
• [ ] #GAP
• [ ] #BACK
• [ ] #BGM
• [ ] #SELECT
• [ ] #SINGLE
• [ ] #DOUBLE
• [ ] #COUPLE

[Anders429]

After finally getting my hands on some actual .msd files from back in the day, there also appears to be an unofficial #MSD tag that creators used to indicate who wrote the file.

[Anders429]

It appears that #REMIXER was also at one time used for tagging creators of .msd files, interchangable with #MSD. Back in the ancient days of Stepmania, the code references this here: https://github.com/stepmania/stepmania/blob/ffd50d90877bf9066db94cdc705efba2da34c977/stepmania/src/Song.cpp#L262

While the code for reading .msd files was never in a functional state until the switch to .dwi, this indicates to me that #REMIXER was commonly used within the community to indicate who stepped a chart. I think it ought to be supported. Perhaps someday I will find a .msd file with this tag in the wild, but that day hasn't come yet.