Generalize lines_longer_than_80_characters

[munificent]

After many years of users asking for it, we're planning to support project-wide configuration of formatter page width (dart-lang/sdk#56863).

If a user sets the page width in their analysis_options.yaml to 100, then a lint that is hardcoded to fire on lines longer than 80 characters isn't super useful anymore. :)

I'm guessing we will want to either rename and change this lint to fire based on the configured page width, or add a new lint that does that and leave this one alone for posterity/backwards compatibility.