UX improvement for adding external JARs

Per survey user feedback, it seems the way to add external JARs is sometimes not obvious or straightforward to the users, especially users who come from IDE background, so creating this issue for discussion

Original words:
“No intuitive way to add external JARs. Just learning Java and had to install Eclipse to figure it out”

这个 issue 涉及此 VS Code Java extension 中的 external-JAR workflow，但没有指明任何文件、测试或入口点。首先检查用户目前如何添加 external JARs，并查看讨论上下文；对于从 IDEs 转来的用户来说，拥有更直观的 workflow 就算完成。