Default option for Vignette

[tildejustin]

Request Description

In vanilla, vignette has no specific toggle. It is instead controlled by the Graphics Mode, with it being on for Fancy or better. In sodium, Vignette is configurable, but unlike the other independent graphics configuration options (leaves, weather), it is only a boolean. I made an implementation of this (tildejustin@31c07e2), but the JSON handling is not very pretty, I can make a PR if it's deemed to be ok. handing the type change of enable_vignette in sodium-options.json is the main issue with implementing this change and maybe it would be better to just rename the field. This request was discussed a bit in the discord (https://discord.com/channels/602796788608401408/651120262129123330/1225850862698758246), just making an issue as a follow-up.