Problem with `SelfInvocationContext`

[SelfInvocationContext](https://github.com/Kotlin/kandy/blob/3de2c8bfb0f196583e9786334557c6513b970723/kandy-api/src/main/kotlin/org/jetbrains/kotlinx/kandy/util/context/SelfInvocationContext.kt) is a special interface that actually works like trait. It has `invoke()` extension function that opens scope with this as a reciever:
```
public operator fun T.invoke(block: T.() -> Unit): T = apply(block)
```

It simplifies DSL creation (see `Axis` or `Legend` API for example), but it complicates work with Kandy in Gradle projects, since user need `invoke` import (and IDEA doesn't even suggest to add this import!).

Start by reading kandy-api/src/main/kotlin/org/jetbrains/kotlinx/kandy/util/context/SelfInvocationContext.kt and compare its use in the Axis and Legend APIs. Reproduce the Gradle usage problem described in the issue, including the missing invoke import and IDEA's lack of an import suggestion. The issue needs a decided direction before a newcomer can tell what change would be complete.

Written by the indexing model from the issue text.