XMonad.actions.search: Add NixOS and home-manager searchEngines

[cafkafk]

• feat(search): add nixos as searchEngine
• feat(search): add homeManager as searchEngine

Description

(I've only skimmed contributing and docs, and haven't tested this, which I will ofc do before changing from draft)

Include a description for your changes, including the motivation behind them.

Checklist

• [x] I've read CONTRIBUTING.md

• [x] I've considered how to best test these changes (property, unit, manually, ...) and concluded: not necessary here, e.g. some other individual search engine entries don't seem to get tested.

• [x] I updated the CHANGES.md file

[geekosaur]

The only thing I'd note is we keep the list of search engines in sorted order so it's easier to check if one you want is present.