Fix_128

[carlocardella]

This PR fixes Issue# Fix for issue #128: Unable to open Gist file with backslash in the filename

Description of the PR: The uri.path value passed from openFistFile (file.ts:164) is json escaped but needs to be HTML encoded; since it is not, vscode fails to find the file and throws the exception described in the issue.

Additional context: Here are the two uri objects as an example, note the different path value

broken:

{
  scheme: "gist",
  authority: "7beed6d4b22a859f2fabc028c2d6dbe5",
  path: "/History\\-258baa23\\entries.json",
  query: "",
  fragment: "",
  _formatted: null,
  _fsPath: null,
}

working:

{
  scheme: "gist",
  authority: "7beed6d4b22a859f2fabc028c2d6dbe5",
  path: "/History%5C-258baa23%5Centries.json",
  query: "",
  fragment: "",
  _formatted: null,
  _fsPath: null,
}

[carlocardella]

@lostintangent I decided to not update the path when it is first added to the Tree because I did not want to inadvertently break other areas that may need it with the json encoding. If you think it is safe to use the HTML encoding instead I can make that change