FIR checker: introduce FunctionChecker alias

and use it to add support diagnostic FUNCTION_DECLARATION_WITH_NO_NAME

compiler/fir/entrypoint/src/org/jetbrains/kotlin/fir/checkers/CommonDeclarationCheckers.kt

@@ -28,6 +28,10 @@ object CommonDeclarationCheckers : DeclarationCheckers() {
         FirSealedSupertypeChecker,
     )
 
+    override val functionCheckers: Set<FirFunctionChecker> = setOf(
+        FirFunctionNameChecker,
+    )
+
     override val propertyCheckers: Set<FirPropertyChecker> = setOf(
         FirInapplicableLateinitChecker,
         FirDestructuringDeclarationInitializerChecker,