Provide an editor to open the diagnostics at project level

Currently when you click an error at project root level in PROBLEMS view, it will throw some error saying Unable to open 'project': File is a directory.

This error is like a bug. When the problems are showed in the PROBLEMS view, the user will expect to open the resource and able to fix it. For the Java language server, the closest project configuration resource file is .classpath, however it's apparently not friendly for user to edit it manually. One idea is the Java language server provides a special web view to show the project structure (such as build path).

首先从 PROBLEMS 视图复现项目级错误，并跟踪 Java language server 如何处理该诊断。检查由 .classpath 表示的项目配置，并确定所提议的项目编辑器或 Web 视图的范围。完成标准是：点击项目级诊断后会打开一个用户友好的资源，支持修复问题，而无需手动编辑 .classpath。