Add `use_string_in_part_of_directives`

[srawlins]

I hereby propose we add use_string_in_part_of_directives.

• The rule is part of Effective Dart.
• The language team is considering banning the old syntax in a future version of Dart.

[devoncarew]

As part of core? recommended?

[srawlins]

I propose for core.

[natebosch]

yes, I vote core. If we could do it for free we'd probably eliminate non-string directives at the language level.

[devoncarew]

Approved for core.