Integration fixes

[mayhem]

After the refactoring of LB Radio and the implementation of the new LB Radio backends, a lot of cleanup and integration was still necessary. In this PR I:

• Finish debugging the new parser with better error messages
• Improve error message handling overall.
• Add support for the country element.
• Integrate new artist and tag search back-ends
• Get rid of old splitter class.
• Update docs to reflect new syntax.

[github-actions]

Test Results

  4 files  ±0    4 suites  ±0   4s ⏱️ ±0s
 50 tests  - 1   50 ✅  - 1  0 💤 ±0  0 ❌ ±0 
200 runs   - 4  200 ✅  - 4  0 💤 ±0  0 ❌ ±0 

Results for commit 5b27377. ± Comparison against base commit 29dd4b2.

This pull request removes 4 and adds 3 tests. Note that renamed tests count towards both.

tests.test_splitter.TestSplitter ‑ test_plist
tests.test_splitter.TestSplitter ‑ test_splitter_basic
tests.test_splitter.TestSplitter ‑ test_splitter_mod_operation
tests.test_splitter.TestSplitter ‑ test_splitter_short

tests.test_parser.TestParser ‑ test_country
tests.test_plist.TestSplitter ‑ test_plist
tests.test_plist.TestSplitter ‑ test_plist_unique

♻️ This comment has been updated with latest results.

[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	45%	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	➖
Summary	22% (4024 / 18000)	0	❌

[amCap1712]

LGTM, Thanks!