Search API

Neovim core does not currently provide any for of API for interacting with the built-in search. 

For this plugin, we require:
- Getting current search results.
- Registering to search events: 
  - navigation via `n`/`N` etc,
  - incremental search updates
  - updates to `v:hlsearch`

Related: https://github.com/neovim/neovim/issues/18879