smartplaylist: change encoding of additional field

[mgoltzsche]

Description

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; see here and here).

I introduced the generation of additional EXTM3U item fields earlier this year and I want to correct that now.
cc @j000

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 am adjusting correspondingly).

To Do

• Documentation. (If you've added a new command-line flag, for example, find the appropriate page under docs/ to describe it.)
• Changelog. (Add an entry to docs/changelog.rst to the bottom of one of the lists near the top of the document.)
• Tests. (Very much encouraged but not strictly required.)

[github-actions]

Thank you for the PR! The changelog has not been updated, so here is a friendly reminder to check if you need to add an entry.