unnecessary_getters_setters should catch wrapping a private final field

[natebosch]

Bad:

class Foo {
  final _foo = 'foo';
  String get foo => _foo; // unnecessary_getters_setters
}

Good:

class Foo {
  final foo = 'foo';
}

[bwilkerson]

I'm not sure why, but we appear to have a separate lint for that (unnecessary_getters).

[pq]

It looks like unnecessary_getters is disabled (see: #57169). Not sure if we want to resuscitate it but I'm happy to discuss.