consider adding strict_top_level_inference

[devoncarew]

We should consider adding a strict_top_level_inference lint, as described here: dart-lang/sdk#59562. Note that this lint does not yet exist.

This lint would warn about every omitted return type, parameter type, and variable type of a top-level declaration or class-like-namespace-level declaration (static or instance member- or constructor declaration), which is not given a type by inference, and which therefore defaults to dynamic.

[goderbauer]

I am in favor of this to get rid of more (implicit) dynamics.