Allow more expanded variables in "basic" contexts

[briancarbone]

It would be useful if this function: https://github.com/neoclide/coc.nvim/blob/master/src/configuration/util.ts#L28

was more like this function: https://github.com/neoclide/coc.nvim/blob/master/src/core/documents.ts#L229

because some language servers (like jdtls) would benefit from being able to put the cache at the root of the project:

    "java": {
      "command": "jdtls",
      "filetypes": ["java"],
      "args": ["-data", "${workspaceFolderBasename}/.cache"],
      "rootPatterns": ["pom.xml", ".git"]
    },

This won't expand because it uses the 'basic' expand function. ${cwd} changes and is evaluated before something like vim-rooter changes the cwd.