URL-encode additional item `fields` within generated EXTM3U playlists instead of JSON-encoding them.
This is because JSON-encoding additional fields/attributes made it difficult to parse the `EXTINF` line but using URL-encoding for these values makes parsing easy (because URL-encoded values cannot contain commas, quotation marks and spaces).
I introduced the generation of additional EXTM3U item fields earlier this year and I want to correct that now.
**Design/definition background:**
Unfortunately, I didn't find a clear definition of how additional playlist item attributes should be encoded - apparently there is none.
Given that item URIs within an M3U playlist can be URL-encoded already, defining the values of additional attributes to be URL-encoded is consistent design.
I didn't find examples of additional EXTM3U item attributes in the web where the attribute value contains a comma, space or quotation mark but examples that specified numeric IDs and URLs as attribute values.
Because the URL attribute examples I found didn't contain URL-encoded characters and because it is more readable and unproblematic for parsing, I've let the attribute URL encoding treat `:` and `/` as safe characters.
**Breaking change:**
While this is a breaking change in theory, in practice it is not since afaik all integrations of the smartplaylist plugin's additional EXTM3U item attribute generation feature (beets-webm3u) work with simple attribute values such as the item ID (numeric) whose formatting/encoding is not affected when changing from JSON to URL-encoding.
In other words the change is backward-compatible with the beets-webm3u plugin (which I'll adjust correspondingly after this beets PR was merged).
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