Implement PsiFile.getAssociatedPsiHelper()

[arksap2002]

Describe the bug
Froward from @Vladislav0Art:

This object may be removed completely if we implement the getPsiHelper as an extension method of PsiFile:

fun PsiFile.getAssociatedPsiHelper(): PsiHelperInterface {
  return when (this) {
            is PsiJavaFile -> JavaPsiHelper()
            // TODO: kotlin later
            else -> throw IllegalArgumentException("Unsupported file type")
        }
}

It is advisable to remove it, as it will reduce the number of entities in the application, especially since PsiHelperGetter is a one-method object.