Use the path of Java Development Kit specified in environment variables for terminals

Hi,
Currently, vscode-java extension get the path to the development kit in the following order

           1) java.home setting in VS Code settings (workspace then user settings)
           2) JDK_HOME environment variable
          3) JAVA_HOME environment variable
          4) Current system path

Both 2) to 4) are something we configure via our OS's environment variable. Can I suggest adding one more?

          5) JDK_HOME, JAVA_HOME, Current system path 
               (specified in the environment variables  for the terminals in VS Code settings (workspace then user settings))

In that case, we will now get to enjoy the variable expansion when specifying the path, and no more hardcoding the path.

issue 中没有指定文件或测试。首先跟踪 extension 如何解析 JDK 路径以及如何读取 VS Code 终端环境设置；完成的标准是：在现有来源之后考虑 workspace 和用户终端变量，并支持变量展开。