VorbisComments: Support `current/total` TRACKNUMBER fields

[Serial-ATA]

It seems to be a pretty common issue where, rather than using the TRACKTOTAL field, the track total will be stored in the TRACKNUMBER field as current/total, just like APE and ID3v2.

Workaround: #431 (comment)

The solution is simple, just need to add a special case for TRACKNUMBER here:

lofty-rs/lofty/src/ogg/read.rs

Line 112 in af0ce91

match key {

Split on '/' if present, and insert both a TRACKNUMBER and TRACKTOTAL item.

[RocketRide9]

I will pick this up. I already have local changes, that seem to work. Will double check it, add test and send a PR