Fix URI creation from path

[aeschli]

In 1.26 we discovered that several extensions probably call the 'vscode.openFolder' command with invalid file URIs:

kalitaalexey.vscode-rust/out/src/components/cargo/cargo_manager.js
  `373,53:                     vscode.commands.executeCommand('vscode.openFolder', uri, true);`
kalitaalexey.vscode-rust/out/src/components/cargo/CargoManager.js
  `192,55:                     vscode_1.commands.executeCommand('vscode.openFolder', uri, true);`

When creating a URI from a file path, always use vscode.URI.file(path), e.g vscode.URI.file('c:\\test')
Use vscode.URI.parse(uriString) when the input string is in the form scheme://authority/path

See microsoft/vscode#57267 for more information or for questions.