Improve ProjectCommand.getMainClasses

[snjeza]

Fixes #882

[wenytang-ms]

the public void main() is the preview feature from JDK 21+
https://www.oracle.com/java/technologies/javase/21-relnote-issues.html

[snjeza]

@wenytang-ms You may want to take a look at https://github.com/microsoft/java-debug/blob/a1f68f4265458e9386bf16d107c2383b68690e43/com.microsoft.java.debug.plugin/src/main/java/com/microsoft/java/debug/plugin/internal/ResolveMainClassHandler.java#L215

[wenytang-ms]

@wenytang-ms You may want to take a look at https://github.com/microsoft/java-debug/blob/a1f68f4265458e9386bf16d107c2383b68690e43/com.microsoft.java.debug.plugin/src/main/java/com/microsoft/java/debug/plugin/internal/ResolveMainClassHandler.java#L215

yes, I see