Summary of changes to JSR 269 for Java SE 18

The proposed changes to JSR 269 for Java SE 18 include:

• Customary changes to support a new platform version, additional enum constant in SourceVersion, etc.
• Convenience methods to Messager for emitting warnings and errors
• Adding getOutermostTypeElement to Elements
• Adding method to map from and element to its JavaFileObject to Elements
• Miscellaneous cleanups