migrate to new @types/vscode, add vsce devDependency

The vscode library is deprecated in favor of @types/vscode.
See https://code.visualstudio.com/api/working-with-extensions/testing-extension#migrating-from-vscode

This also add vsce as a devDependency, so you don't need to install
it globally. Instead you can run it via npm run vscepackage. This
simplifies the README (or future "hacking" instructions when the
extension is published)

Author: roberth

README.md

@@ -61,9 +61,8 @@ Install the VS code extension (see https://code.visualstudio.com/docs/setup/mac
 
 1. `cd extension/`
 2. `npm ci`
-3. `npm install vsce --global` (may require `sudo`)
-4. `vsce package`
-5. `code --install-extension ghcide-0.0.1.vsix`
+3. `npm run vscepackage`
+4. `code --install-extension ghcide-0.0.1.vsix`
 
 Now openning a `.hs` file should work with `ghcide`.