Move config to lua setup

[mvllow]

Idea

local palette = require('rose-pine.palette')
require('rose-pine').setup({
    variant = 'dawn',
    colors = {
        comment = palette.foam,
    }
})

Unfortunately, I am struggling to get this working properly without a dependency loop. With my attempts, config.lua requires palette.lua to set default colours and palette.lua requires config.lua for the variant to return the correct palette.

[mvllow]

This is currently implemented on the canary branch. API may change before release.