Tangara podcast support

[Slord6]

Issue to track the bigger pieces that need to fall together to have a functional user loop as outlined in #24.

Tangara is currently limited on the playlists it supports - comment here and here, quoted for completeness:

The way to create and play them at the moment is to create a file on the SD card with a ".m3u8" or ".playlist" extension,
containing the absolute filepaths (relative to the SD card root) of each track on a new line, ie:

Music/SomeArtist/AnAlbum/01 Some Track.flac
Music/AnotherArtist/AnotherAlbum/02 Some other track.opus

etc.

These can then be played by selecting them in the file browser.

To support audio books using this format, we'd still need to:

Add support for m3u comments. Whilst we don't need to necessarily do anything with the comments, we should probably not treat them as filepath entries.

• Add support for relative filepaths based on the location of the m3u file

• Allow for playing these files either through the file browser, or some kind of indexing

Track positions are now tracked, but "saving the position doesn't necessarily mean we're doing anything with that position" - so as it stands we'd have to serial to the device and grab scrobbling info that way via Lua which isn't ideal.

Changes that need to happen:

• Tangara support for comments in playlist files
• Tangara exposes the status of which files have and haven't been played in any given playlist - scrobbling. Tracked here
• We support mapping the local path to what the path will be on-device. We currently use relative-to-playlist-file paths, but we need relative-to-root-of-device paths. (i.e. prefix with /Podcast/
• We move the right files to the right place (as per Tangara docs for long-form audio (this might not exist yet)) on the device
• We flag if file types will be unsupported - see docs

[Slord6]

Centralised playlist docs now exist for the tangara side; https://cooltech.zone/tangara/docs/playlists/