Consider adding `always_declare_return_types`

[vincevargadev]

Describe the rule you'd like to see added and to what rule set

I'd like to see always_declare_return_types included in the rules.

To what rule set? Ideally core (but I'll take whatever I can get 😸).

If there is a good reason not to include this rule, it would be great to document that, I couldn't find why it isn't included.

Additional context

• It is included in the "community" passsy/dart-lint (lint)
• It was included in google/pedantic

I've seen this rule mentioned in the following issues, but I didn't see any of these issues actually addressing why this rule is no longer included, it looks to me that it was simply dropped without any explanation

• https://github.com/dart-lang/lints/issues?q=is%3Aissue+always_declare_return_types