[FIR] Make FirImport an abstract class

This change is necessary in order to make `FirElement` abstract class. To do it we need to remove all `accept` and `transform` methods from fir interfaces. So we have to make sure that all fir element implementations have their `accept` and `transform` implementations in their super classes. In particular this change makes `FirImport` abstract class in order to preserve `accept` and `transform` methods after removing them from fir interfaces.

compiler/fir/tree/tree-generator/src/org/jetbrains/kotlin/fir/tree/generator/NodeConfigurator.kt

@@ -509,6 +509,7 @@ object NodeConfigurator : AbstractFieldConfigurator<FirTreeBuilder>(FirTreeBuild
             +booleanField("isAllUnder")
             +field("aliasName", nameType, nullable = true)
             +field("aliasSource", sourceElementType, nullable = true)
+            shouldBeAbstractClass()
         }
 
         resolvedImport.configure {