Add the VNDB API as a `Game` source for visual novels

[Senyksia]

This adds VNDB as a new API source for visual novels. The API documentation and endpoint can be found here. All GameModel fields are populated, and the SFW flag is supported[^1].

Spurred primarily by #164, and Steam typically having a blind spot for visual novels.

Some implementation details for RFC:

• I've set the number of results requested per search query to 20 (from the default 10), which I believe matches our other sources.
• The GameModel.genres field is populated with content tags that have been scored as significant, and aren't spoilers. This is my interpretation of what most people will actually want, but we can always spit out every tag.
• Because VNDB can return a partial release date (e.g. 2024-10), the plugin dateFormatter will end up coercing these dates to an erroneous specificity (e.g. 01-10-2024). This is relatively minor.

[^1]: VN entries with no official SFW releases and no content toggle are filtered out.