[Feature Request] Expose source paths setting in VS Code for unmanaged folders

[jdneo]

Currently we have several classpath related settings for the unmanaged folders:

• Default JDK: java.configuration.runtimes
• Output path: java.project.outputPath (under release)
• Dependencies: java.project.referencedLibraries

The missing part is a setting to specify the source paths which originally comes from: microsoft/vscode-java-dependency#412

Though we now have a command Add/Remove to Java Source Path, users are still not aware of this capability. So adding a new setting can better expose such capability.