improving the Searcher JS module

[fz-ryanbigg]

Hello, I'd like to propose a change to the Searcher.js module at lib/rdoc/generator/template/json_index/js/searcher.js, to have this switch to using Fuse.js for its full text searching and highlighting capabilities. That part I can handle in a PR.

What part I got stuck on was how to test that this new implementation would be working as well (or, ideally, better) than the previous one. Are there some test cases that this should be passing within the RDoc project itself or some real-world cases that I could try out?