Restore and deprecate Function{n}/ExtensionFunction{n} classes for easier migration

Users' Java code will not break in common cases (when passing functions to
Kotlin), and deprecation warnings will be reported.

Provide an inspection with a quick fix which allows to replace deprecated
function class usages to the new classes. Include this fix to the "code
cleanup" action

libraries/tools/runtime/pom.xml

@@ -46,6 +46,7 @@
                                 <copy todir="${basedir}/target/copied-sources">
                                     <fileset dir="${basedir}/../../../core/builtins/src"/>
                                     <fileset dir="${basedir}/../../../core/runtime.jvm/src"/>
+                                    <fileset dir="${basedir}/../../../core/functions.migration/src"/>
                                 </copy>
                             </target>
                         </configuration>