Add some topics to pubspec.yaml

[g123k]

Hi everyone,

It's not a well-known feature, but in the pubspec.yaml, we can provide a topics section:

topics:
 - interop
 - ffi
 - codegen

We can find them on this page and obviously on the dedicated page (like this one).

I don't know exactly which topic we can use, but it's still a way to improve our visibility.