Apply bundler to the extension

[alanz]

vsce package says

This extension consists of 2753 separate files. For performance reasons, you should bundle your extension: https://aka.ms/vscode-bundle-extension

[halhenke]

Hey - I'm happy to take this - have something working with parceljs but for some reason it only packages properly when vsce is used with yarn (vsce package --yarn). Will try to get something working with npm if possible but otherwise will make a PR.