Add convenience method for `MapProperty` that was introduced in 5.1

Similar convenience like the other *property() methods

[x] Base the PR against the develop branch
[x] Make sure that all commits are signed off to indicate that you agree to the terms of Developer Certificate of Origin.
[ ] Provide integration tests to verify changes from a user perspective
[x] Provide unit tests to verify logic
[x] Ensure that tests pass locally: ./gradlew check --parallel

gradle / kotlin-dsl-samples