Enable page heading searchable via `SearchIndexPlugin`

[origami-z]

Currently the plugin produces a search index with all content on a page as string for a route.

mosaic/packages/store/src/types/searchIndex.ts

Lines 1 to 5 in 7379927

	type SearchDataPage = {
	title: string;
	content: string[];
	route: string;
	};

This doesn't allow the use case to search for specific page section heading and navigate to it.

Potential solution - it may mean a similar data like below

type SearchDataPage = {
  title: string;
  sections: {
    heading?: string;
    id?: string;
    content: string[];
  }[];
  route: string;
};

This likely means 2 different parts of change, first when indexing content, it will need to differentiate heading (e.g. AST type: "element" andtagName: "h1") vs other content

mosaic/packages/plugins/src/SearchIndexPlugin.ts

Lines 114 to 127 in 7379927

	visit(
	tree,
	(node: Node) => node.type === 'text' || node.type === 'code',
	(node: LeafNode) => {
	const segments = [...segmenter.segment(node.value)].map(segment => segment.segment);
	segments.forEach(segment => {
	if (maxLineLength) {
	sentences.push(segment.slice(0, maxLineLength));
	} else {
	sentences.push(segment);
	}
	});
	}
	);

I'm not sure how to deal with getting correct id, given computing id from heading is not enough given potential collision. This could mean a different lifecycle method is needed for this to work?