add toggleterm highlights and initial bufferline highlights

[mvllow]

closes #76

[mvllow]

This adds a starting point for supporting plugins in which the user sets their own highlights. Included are bufferline and toggleterm:

use({
	'akinsho/bufferline.nvim',
	event = 'ColorScheme',
	config = function()
		local highlights = require('rose-pine.plugins.bufferline')
		require('bufferline').setup({
			highlights = highlights,
		})
	end,
})
use({
	'akinsho/toggleterm.nvim',
	event = 'ColorScheme',
	config = function()
		local highlights = require('rose-pine.plugins.toggleterm')
		require('toggleterm').setup({
			highlights = highlights,
		})
	end,
})

Notice that event = 'ColorScheme' is necessary when Rosé Pine uses the dawn variant.

For anyone wanting to contribute, please reach out here. Ideally we would have a PR per plugin – simply use this PR for reference 😌

[will]

Thanks!