Support import-only files with include rules (#300)

* Support import-only files with include rules

This enables migration of some nested/late imports that the migrator
couldn't previously handle by first wrapping the file that's depended on
via nested import in a mixin and then `@include`ing that mixin in the
import-only file.

* Allow multiple includes and arbitrary arguments

This adds a `NamespacedSerializer` utility class, which allows us to
copy the include rules with arbitrary arguments while still ensuring
that we use the right namespaces and add any additional use rules when
migrating.

* Run formatter

* Use patching instead of serialization

Support `@forward` rules with `--migrate-deps` (#299)

Increase the Node.js version we test and release against (#295)

See #287

Use a defined Node version (#294)

Get the Node and npm versions necessary for trusted publishing (#293)

Enable the permissions necessary to do npm trusted publishing (#292)

See #287

Use trusted publishing rather than tokens to deploy to npm (#291)

See #287

Add a changelog entry for 2.5.1 (#290)

Merge pull request #288 from sass/fix-ci

Fix `dart pub add` invocation

Improved late import hoisting (#282)

* Improved late import hoisting

While 2.4.0 allowed the migrator to hoist late imports that did not emit
CSS themselves, this additionally allows it to hoist them above safe at
rules in the same file, even if the imports themselves do actually emit
CSS.

* Code review