Inner java class miss the `inner` modifier in signature

[atyrin]

public class JavaClassWithNested {
    public class Inner{}
    public static class Nested{}
}

Will generate documentation like

The signature for class Inner should contain inner keyword.

[IgnatBeresnev]

Notes for those looking to contribute:

The inner keyword is rendered correctly for Kotlin inner classes, so there's support for it, but it's missing from our Java source code parser.

Java's inner classes already have the static modifier in the model, but it looks like KotlinSignatureProvider renders only Kotlin-specific ones (ExtraModifiers.KotlinOnlyModifiers), and Inner is one of them.

So we need to add the ExtraModifiers.KotlinOnlyModifiers.Inner additional modifier to the parsed Java class if it's static:

extra = PropertyContainer.withAll(
    AdditionalModifiers(setOf(ExtraModifiers.KotlinOnlyModifiers.Inner).toSourceSetDependent())
)

You can check for the Java's static modifier by using psi.hasModifier(JvmModifier.STATIC)

The code for parsing Java classes can be found here.

See CONTRIBUTING.md and Developer Guides to get started.