feat: add opensearch endpoints powered by quicksearch

[LeoMcA]

Part of mdn/fred#1079

Ports quicksearch in fred into the cloud function, for use from OpenSearch:

• adds a "suggestions" route to return quicksearch results
• differs from fred in appending a section of breadcumb to the result to distinguish between pages with the same title where necessary
• adds a "go" route, to redirect to search, or the actual page if the user has selected a suggestion
• takes over generating the opensearch.xml from fred, in order to control locales: opted for simplicity here, adding the locale as a query, so the added search engine matches the locale of the page the user adds it from, rather than doing accept-language negotiation at query time
• adds extensive tests
• logic ported from fred is clearly commented as so, to make unifying in the future easier
• adds BASE_URL_MAIN so it's possible to test this locally, with fred and the cloud function running on different ports

[globau]

If the search backend fails, this will return HTTP/200 with an empty result that is cached for an hour.

Either return a http error, or don't cache when an error occurs.

[globau]

This is different from fred, which does:

return indexResults.map((i) => i && (index.items || [])[i]).filter(Boolean);

Which skips the first element. I'm not sure which is correct; it seems non-accidental in fred and has been in prod for a while.

In any event, make sure the funcs in the two repos match, or add a comment explaining the divergence.