JSPF Track identifiers should be a list

[amCap1712]

See #115 for rationale.

@mayhem attempted to fix the issue in #138 again, but I got confused and reverted it, fixing it now.

[github-actions]

Test Results

  4 files  ±0    4 suites  ±0   4s ⏱️ ±0s
 50 tests ±0   50 ✅ ±0  0 💤 ±0  0 ❌ ±0 
200 runs  ±0  200 ✅ ±0  0 💤 ±0  0 ❌ ±0 

Results for commit 2c41fa4. ± Comparison against base commit 9dc5882.

[github-actions]

Package	Line Rate	Complexity	Health
.	33%	0	❌
content_resolver	0%	0	❌
content_resolver.formats	6%	0	❌
content_resolver.model	0%	0	❌
external	39%	0	❌
listenbrainz	46%	0	❌
musicbrainz	71%	0	➖
patches	25%	0	❌
patches.lb_radio_classes	0%	0	❌
tools	74%	0	➖
.	33%	0	❌
content_resolver	0%	0	❌
content_resolver.formats	6%	0	❌
content_resolver.model	0%	0	❌
external	39%	0	❌
listenbrainz	46%	0	❌
musicbrainz	71%	0	➖
patches	25%	0	❌
patches.lb_radio_classes	0%	0	❌
tools	74%	0	➖
.	33%	0	❌
content_resolver	0%	0	❌
content_resolver.formats	6%	0	❌
content_resolver.model	0%	0	❌
external	39%	0	❌
listenbrainz	46%	0	❌
musicbrainz	71%	0	➖
patches	25%	0	❌
patches.lb_radio_classes	0%	0	❌
tools	74%	0	➖
.	33%	0	❌
content_resolver	0%	0	❌
content_resolver.formats	6%	0	❌
content_resolver.model	0%	0	❌
external	39%	0	❌
listenbrainz	45%	0	❌
musicbrainz	71%	0	➖
patches	25%	0	❌
patches.lb_radio_classes	0%	0	❌
tools	74%	0	➖
Summary	22% (4028 / 18084)	0	❌