API page cross-linking between JS/TS and Dart

[chalin]

Individual API pages, like most doc pages, have a language selection dropdown at the top. Dart API pages will be supported soon (#1880, #1888) and the mapping between JS/TS and Dart API page entries is not one-to-one, and so the current cross-referencing scheme (which just changes the language but keeps the rest of the API page URL the same), needs to be replaced.

Note that this impacts JS/TS and Dart.

cc @kwalrath

[chalin]

I can think of a new xref scheme that will work ~80% of the time. For now we might want to go for that and for the remaining 20% just say that no correspondence can be found.