Provide warning on config files that are not currently configured for the project

[evie-lau]

Following up on discussion in #213 (comment), with the change to allow LCLS support for all bootstrap.properties and server.env files, it is possible to start editing files that have not yet been configured for the Liberty project to use.

Suggestion for enhancement to provide a warning if the currently edited file is not configured for Liberty to use.

Needs design discussion

• Diagnostic probably won't work because there is no particular text to highlight.
• Could possibly provide a toast/popup notification.

This would also be affected by the fact that liberty-plugin-config.xml may not have been generated yet in a new or cleaned project, which may raise the warning until the project is built (liberty:create). An idea has been raised to possibly automatically run a lightweight mvn/gradle goal to generate the liberty-plugin-config.xml