[Feature Request] Support tilde expansion in config

  "markdownlint.config": {
    // tilde is not supported
    "extends": "~/.config/markdownlint/config"
  },

Currently ~ is not supported in vscode-markdownlint configuration, but supported in default settings like python.defaultInterpreterPath, etc. Hope this feature could be supported.

Environments

• OS: macOS 10.14.6
• VSCode: 1.57.1
• vscode-markdownlint: 0.43.2

This should work in the next update, via https://github.com/DavidAnson/markdownlint/commit/ffc4d569182a116c0233b2efc97353364c470702.