feat(mandelbrot): improve variants searchability

[mihkeleidast]

Should resolve #962.

Made some improvements to the search as well.

• Mainly, instead of directly looking for text matches in DOM, I now look for <mark> elements. That's because the highlighting via mark.js takes place before traversing the navigation to open/close things. Since mark.js actually does the search as well, it should be fine to use its results instead of trying to come up with my own.
• The previous change has the side effect of making search a bit fuzzier. E.g. if searching for multiple words, the results to not have to include all keywords, instead only one.
• Search now looks at the navigation group title as well. That is partially because of the issue highlighted in Searching hides component variants in split variant navigation #962, but additionally because of the functionality to pull collections to root. This change should make searching for collections behave the same for root and non-root collections. Example: if a root collection is labelled "root", but does not have any subitems that include the word "root", it should be kept visible in the navigation.
• Search now also looks at variant collections of not-current components and shows/hides them appropriately. This should fix the issue brought up in #775 (comment), which was really the previous behavior from pre-split navigation.