Set default anonymous source to nearest

[johannesodland]

The progress demo is currently throwing a "TypeError: Invalid ScrollTimeline Source Type".

This happens because

1. parseAnonymousScrollTimeline() does not set a default source
2. getAnonymousSourceElement() requires a source, and will throw an error if one is not set.

Adding 'nearest' as a default anonymous source in getAnonymousScrollTimelineOptions(), the same place a default value for axis is set.

scroll-timeline/src/scroll-timeline-css-parser.js

Lines 465 to 481 in db93642

	parseAnonymousScrollTimeline(part) {
	const anonymousMatch = RegexMatcher.ANONYMOUS_SCROLL_TIMELINE.exec(part);
	if(!anonymousMatch)
	return null;
	const value = anonymousMatch[VALUES_CAPTURE_INDEX];
	const options = {};
	value.split(" ").forEach(token => {
	if(TIMELINE_AXIS_TYPES.includes(token)) {
	options['axis'] = token;
	} else if(ANONYMOUS_TIMELINE_SOURCE_TYPES.includes(token)) {
	options['source'] = token;
	}
	});
	return options;
	}

scroll-timeline/src/scroll-timeline-css-parser.js

Lines 117 to 129 in db93642

	getAnonymousScrollTimelineOptions(timelineName, target) {
	const options = this.anonymousScrollTimelineOptions.get(timelineName);
	if(options) {
	return {
	anonymousSource: options.source,
	anonymousTarget: target,
	source: getAnonymousSourceElement(options.source, target),
	axis: (options.axis ? options.axis : 'block'),
	};
	}
	return null;
	}

scroll-timeline/src/scroll-timeline-base.js

Lines 493 to 504 in db93642

	export function getAnonymousSourceElement(sourceType, node) {
	switch (sourceType) {
	case 'root':
	return document.scrollingElement;
	case 'nearest':
	return getScrollParent(node);
	case 'self':
	return node;
	default:
	throw new TypeError('Invalid ScrollTimeline Source Type.');
	}
	}