Add collection_methods_unrelated_type to core

[srawlins]

This new rule guards against a super-set of code which iterable_contains_unrelated_type and list_remove_unrelated_type guard against. These two rules are in core.yaml, and could be replaced by collection_methods_unrelated_type.

https://dart-lang.github.io/linter/lints/collection_methods_unrelated_type.html

[devoncarew]

@natebosch, @pq - thoughts on adding this rule?

cc also @mit-mit

[lrhn]

All for adding this, with the same reasoning as the existing related lints (which then become unnecessary, #118).

[devoncarew]

Approved for core.