Lunr JS search support

[AjaniBilby]

I know you already have quite a lot of features already in the pipeline for new search engines let alone other features. However I'd like to recommend adding Lunr support if possible; as it is a open source search engine written in JS which would then have easy drop in support depending on how your backend is written.

It's used in Material.io MkDocs as an example of it in another open source similar project.

[NGPixel]

External search engines are being deprecated in v3.