Allow the version to link to the versioned pages on api.dart.dev

[athomas]

Currently, dartdoc generates the version as plain text. On api.dart.dev, it would be nice if the version was a link to the versioned (permalink) variant of the page:

https://api.dart.dev/dart-ffi/dart-ffi-library.html -> Click the version number (the current stable in this case) https://api.dart.dev/stable/3.5.4/dart-ffi/dart-ffi-library.html (same page, but will not change when new versions are published)

https://api.dart.dev/beta/latest/dart-ffi/dart-ffi-library.html -> Click the version number (the current beta in this case) https://api.dart.dev/beta/3.6.0-334.4.beta/dart-ffi/dart-ffi-library.html (same page, but will not change when new versions are published, note: unlike stable links, this page will expire at some point)

[athomas]

A fancier variant would be a version selector as requested here: https://github.com/dart-lang/api.dart.dev/issues/86