FIR: record and serialize the modifier "fun" for functional interface

compiler/fir/raw-fir/light-tree2fir/src/org/jetbrains/kotlin/fir/lightTree/converter/DeclarationsConverter.kt

@@ -382,6 +382,7 @@ class DeclarationsConverter(
                     isCompanion = modifiers.isCompanion() && classKind == ClassKind.OBJECT
                     isData = modifiers.isDataClass()
                     isInline = modifiers.isInlineClass()
+                    isFun = modifiers.isFunctionalInterface()
                 }
 
                 buildRegularClass {

compiler/fir/raw-fir/light-tree2fir/src/org/jetbrains/kotlin/fir/lightTree/fir/modifier/Modifier.kt

@@ -78,6 +78,10 @@ class Modifier(
         return classModifiers.contains(ClassModifier.COMPANION)
     }
 
+    fun isFunctionalInterface(): Boolean {
+        return classModifiers.contains(ClassModifier.FUN)
+    }
+
     fun hasOverride(): Boolean {
         return memberModifiers.contains(MemberModifier.OVERRIDE)
     }

compiler/fir/raw-fir/light-tree2fir/src/org/jetbrains/kotlin/fir/lightTree/fir/modifier/ModifiersEnumSet.kt

@@ -11,7 +11,8 @@ enum class ClassModifier {
     DATA,
     INLINE,
     INNER,
-    COMPANION
+    COMPANION,
+    FUN
 }
 
 enum class MemberModifier {

compiler/fir/raw-fir/psi2fir/src/org/jetbrains/kotlin/fir/builder/RawFirBuilder.kt

@@ -670,6 +670,7 @@ class RawFirBuilder(
                     isCompanion = (classOrObject as? KtObjectDeclaration)?.isCompanion() == true
                     isData = classOrObject.hasModifier(DATA_KEYWORD)
                     isInline = classOrObject.hasModifier(INLINE_KEYWORD)
+                    isFun = classOrObject.hasModifier(FUN_KEYWORD)
                 }
                 withCapturedTypeParameters {
                     if (!status.isInner) context.capturedTypeParameters = context.capturedTypeParameters.clear()