Use a custom set of linter rules

[triallax]

lint hasn't been updated with Flutter 3 lints in a while. Possible alternatives:

• very_good_analysis: seems to be pretty strict, even more so than lint
• lints: "official" lint rules package, seems to not be as strict as the other two

[triallax]

How about having our own set of custom rules? This will give us complete control over the set of lints we enable, but it means we'll have to be on the lookout for new lints to possibly add each Flutter release. It'll be somewhat tedious without #244 though.