Generate compatible AST for Kotlin and Java

[tsantalis]

RefactoringMiner supports the matching of Java code to Kotlin.
Migrating Java to Kotlin is a common scenario in IntelliJ and Android projects.

Case studies:
JetBrains/intellij-community@2f6c8c0
square/okhttp@34bb125

The generated AST diff is not aesthetically pleasing, as currently the AST trees for Java and Kotlin are incompatible.

We need to generate a common AST representation when generating a diff between Java and Kotlin.

My idea is the implement a org.jetbrains.kotlin.psi.KtVisitor which generates a gumTree Tree in the same way that
org.refactoringminer.astDiff.visitors.JdtWithCommentsVisitor generates a gumTree Tree.

[tsantalis]

This is the current state of the generated diff for the same case.

AST nodes additionally matched:

• Infix expression operators
• Assignment operators
• Variable declaration =

There is still space for improvement, but it seems possible to align the trees despite their structural differences.

[tsantalis]

This is the current state of the generated diff for the same case.

[tsantalis]

This is the current state of the generated diff for the same case.