Check if our search engine customization still works after ESR 140 transition

Between ESR 115 and ESR 128, upstream completely refactored their search engine configuration.

As a result, they decided engine IDs could use dash only for the locale, and added a list of exceptions for legacy engines that already had a dash in their name (NON_SPLIT_ENGINE_IDS in toolkit/components/search/SearchService.sys.mjs).

In Firefox 131, they removed that list (568c4b44). We should investigate if this has consequences for us (for ddg-onion and startpage-onion).

In addition to that, that commit also refactors the rest of _fetchEngineSelectorEngines.

Notice that search engine webextensions have been removed with Bug 1885953 in Firefox 135 (#43111 (closed)).

The commit in 130 before all this mess was 5613de84.