API result caching does not seem to work

[padarom]

https://github.com/josefalcon/wanikanify/blob/2bde4314cd5504d9c913fd266195a50c748b2186/js/content.js#L186-L192

The WaniKani API is requested, but the result does not get stored in local storage. It'd likely be sufficient to replace the call with cacheVocabList(await tryWaniKani(apiKey)), but I haven't tried yet.

Especially with a migration to API v2 (#12), where we need to request 3 endpoints and potentially multiple pages each, while simultaneously being limited to 60 requests per minute, caching seems necessary.