Make onKeyDown logic more readable

[jamesmaa]

The logic in search-display-controller.js's onKeyDown logic is difficult to reason about. Task is to either simplify this logic or make it easier to read/reason about. A test or some way to verify it preserves behavior is very ideal

Related discussion: https://github.com/themoeway/yomitan/pull/914/files#r1597534169

[brandonrainey]

Thanks for putting up these good-first-issues! I have a easier to reason about version made, just trying to figure out writing a test for it.