Language Detector task

[craiglabenz]

Description

Adds the LanguageDetector.detect(...) task.

Checklist

Before you create this PR confirm that it meets all requirements listed below by checking the relevant checkboxes ([x]). This will ensure a smooth and quick review process. Updating the pubspec.yaml and changelogs is not required.

• [x] I read the Contributor Guide and followed the process outlined there for submitting PRs.
• [ ] My PR includes unit or integration tests for all changed/updated/fixed behaviors (See Contributor Guide).
• [ ] All existing and new tests are passing.
• [x] I updated/added relevant documentation (doc comments with ///).
• [x] I read and followed the Flutter Style Guide.
• [x] I signed the CLA.
• [x] I am willing to follow-up on review comments in a timely manner.

[craiglabenz]

Apologies for taking a minute to fix that merge conflict, @Piinks, but this should now be ready.