fix: handle UTF BOM in lyrics and playlist files (#4637)

* fix: handle UTF-8 BOM in lyrics and playlist files

Added UTF-8 BOM (Byte Order Mark) detection and stripping for external lyrics files and playlist files. This ensures that files with BOM markers are correctly parsed and recognized as synced lyrics or valid playlists.

The fix introduces a new ioutils package with UTF8Reader and UTF8ReadFile functions that automatically detect and remove UTF-8, UTF-16 LE, and UTF-16 BE BOMs. These utilities are now used when reading external lyrics and playlist files to ensure consistent parsing regardless of BOM presence.

Added comprehensive tests for BOM handling in both lyrics and playlists, including test fixtures with actual BOM markers to verify correct behavior.

* test: add test for UTF-16 LE encoded LRC files

Signed-off-by: Deluan <deluan@navidrome.org>

---------

Signed-off-by: Deluan <deluan@navidrome.org>

tests/fixtures/bom-test.lrc

@@ -0,0 +1,4 @@
+[00:00.00] 作曲 : 柏大輔
+NOTE: This file intentionally contains a UTF-8 BOM (Byte Order Mark) at byte 0.
+This tests BOM handling in lyrics parsing (GitHub issue #4631).
+The BOM bytes are: 0xEF 0xBB 0xBF

tests/fixtures/playlists/bom-test.m3u

@@ -0,0 +1,6 @@
+#EXTM3U
+# NOTE: This file intentionally contains a UTF-8 BOM (Byte Order Mark) at the beginning
+# (bytes 0xEF 0xBB 0xBF) to test BOM handling in playlist parsing.
+#PLAYLIST:Test Playlist
+#EXTINF:123,Test Artist - Test Song
+test.mp3