Add @vscode/vsce to devDependencies

[samestep]

Currently, running

npm install
npm run package

fails if one does not already have @vscode/vsce globally installed for some reason. This PR adds it to package.json so the above commands work as expected.

[jryans]

While I do think most people planning to develop a VS Code extension would already have it installed globally, it doesn't hurt to list it explicitly like this in case they do not.

[samestep]

While I do think most people planning to develop a VS Code extension would already have it installed globally, it doesn't hurt to list it explicitly like this in case they do not.

I suppose, since I often see this in repos for VS Code extensions, but I work on several VS Code extensions and generally prefer not to install npm packages globally.

Anyways, thanks for merging this!