Search: Add taxonomy terms to search results

#### Is your feature request related to a problem? Please describe.

Currently, search results include posts only. If possible, we add taxonomy terms to results too.

#### Describe the solution you'd like

Lets say I have custom taxonomy „cars“ and it contains terms like „bmw“, „audi“,“mercedes“,“mercedes-benz“ …

When user on my site is searching for query „mercedes“, it will show him (besides the found posts) links to the terms „mercedes“ and “mercedes-benz“.

So basically it would index the terms and use them for search too.

#### Describe alternatives you've considered

#### Additional context

From 2741810-zen

The issue names no files, tests, or entry points. Trace how WordPress posts and custom taxonomy terms are indexed and rendered in search results, then define how matching terms such as “mercedes” and “mercedes-benz” should appear alongside posts and verify the behavior with search coverage.

Written by the indexing model from the issue text.