Improve implementation of index

[singaround]

The current way the index is done is slightly odd, setting two variables that the include relies on:

songbook/pages/index.njk

Lines 12 to 14 in 0412aca

	{% set indexlist = collections.songsByFirstline %}
	{% set indexItem = "songLine" %}
	{% include "components/indexlist.njk" %}

This feels like it might be an ideal place for to use eleventy shortcodes https://www.11ty.io/docs/shortcodes/

The other option would be to set an indexKey property or similar when creating the collections:

songbook/.eleventy.js

Lines 82 to 85 in 0412aca

	return item.data.chorusLine != null;
	}).sort(function (a,b) {
	return a.data.chorusLine.localeCompare(b.data.chorusLine)
	});

(similar to the way we set songNumber when iterating over all songs).

This would at least skip the second magic variable.