Add support for workspace specific java.home that use vscode variables

In the workspace specific settings.json,

Does not work:
"java.home": "${workspaceRoot}/java/jdk64"

Works:
"java.home": "D:/dev/proj-1/java/jdk64" works.

Environment

• Operating System: Windows
• JDK version: 8
• Visual Studio Code version: 1.31.1
• Java extension version: 0.38.0

Steps To Reproduce

1. Open a workspace
2. Create a folder named .vscode
3. Create a file in the folder named settings.json
4. Try the setting above that does not work by adding it to settings.json

首先，在 Windows 上使用 JDK 8，通过工作区专用的 .vscode/settings.json 文件和 ${workspaceRoot}/java/jdk64 值重现该行为。将其与可正常工作的绝对路径进行比较，然后跟踪 Java extension 如何读取 java.home 并解析 VS Code 变量。当工作区设置接受该变量并使用预期的 JDK 时，即表示完成。