[search] Refactor: make the search code more testable.

[jayaddison]

Feature or Bugfix

• Refactoring

Purpose

• Make it easier to write and run tests for the HTML search JavaScript code by extracting functions that can be called individually to parse queries and perform searches, without running any display/DOM-related logic.

Detail

• Extract Search._parseQuery to transform a query into search terms.
• Extract Search._performSearch to run a query and return results using search term arguments.
• Update the test suite to use the newly-extracted functions.

Relates

• Builds upon [tests] JavaScript: extract searchindex.js-format test fixtures. #12102.
• Makes it easier to write JavaScript test coverage for HTML Search: omit anchor reference from document titles in the search index. #12047.

[jayaddison]

Awaits possible merge of #12102; moving into draft status until then.

[jayaddison]

Awaits possible merge of #12102; moving into draft status until then.

It seems I've unintentionally merged most of this into #12102. I'll close this and merge the remainder too, so that there's only one PR to review. It could make sense to separate some of it out again, but to begin with a single PR should be less confusing I expect.