文档来自https://code.visualstudio.com/docs/editor/variables-reference
预定义的变量有：
- ${workspaceFolder} - the path of the folder opened in VS Code
- ${workspaceFolderBasename} - the name of the folder opened in VS Code without any slashes (/)
- ${file} - the current opened file
- ${relativeFile} - the current opened file relative to
workspaceFolder - ${fileBasename} - the current opened file's basename
- ${fileBasenameNoExtension} - the current opened file's basename with no file extension
- ${fileDirname} - the current opened file's dirname
- ${fileExtname} - the current opened file's extension
- ${cwd} - the task runner's current working directory on startup
- ${lineNumber} - the current selected line number in the active file
- ${selectedText} - the current selected text in the active file