Warn of properties being overridden in property files

Per the 3rd bullet in the comment on issue 181, an additional nice feature would be yellow underlining of properties that are being overridden (or are overriding) the same property from another config source. For example, if quarkus.http.port is in both the microprofile-config.properties and application.properties, then the yellow underline message in microprofile-config.properties should be that it is being overridden by application.properties, and in application.properties, there should be a message that it is overriding the same property in microprofile-config.properties.

It would be nice to take this to the "next level", notifying of duplicate properties defined in system properties ("-D"), environmental variables, and application.yaml as well.

This requirement does not include custom or dynamic config sources, although that would admittedly be a nice touch.

Start with the third bullet in the comment on issue 181, then locate the extension entry point that validates or underlines configuration properties. Define completion as diagnostics for duplicate properties across microprofile-config.properties, application.properties, system properties, environment variables, and application.yaml, while excluding custom or dynamic sources.