Add support for `Property` `property value` assignment in Groovy DSL and document it

[asodja]

When doing a Provider API migration a Groovy DSL behavior change with Property. This stops working:

testTask.configure {
    options {
        configFailurePolicy "continue"
    }
}

This still works:

testTask.configure {
    options {
        configFailurePolicy = "continue"
    }
}

A possible solution:

Leonard Brünings Could you add a call() extension method to Property that would internally delegate to set? Then the first variant should keep working.

---

• [x] Add support for property value assignment -> merged to EAP branch https://github.com/gradle/gradle/pull/25963
• [ ] Document possible breaking changes, e.g.:
  • https://github.com/gradle/gradle/issues/30471
  • https://github.com/gradle/gradle/issues/30359

[asodja]

A fix was merged to EAP branch https://github.com/gradle/gradle/pull/25963.

Note: We should also add possible breaking change documentation.

[mlopatkin]

I wonder if we should support that in the long term:

• It doesn't work everywhere (not for lists and maps)
• it has weird corner cases (e.g. negative integers)
• it isn't consistent with Kotlin/Declarative

Maybe we can introduce it as deprecated?

[asodja]

Maybe we can introduce it as deprecated?

This sounds like a good idea to me.