Add a webext packaging target to the build script

Run npm run pack-webext -- 0.0.x

I imagine that'll do something like,

git clean -x -d -f webext/
require('webext/manifest.json').version = argv[1];
git commit -m "bump version to ${argv[1]}"
git tag argv[1]
git push --tags
npm run webext
zip webext

See comment:6:ticket:31087 for the original suggestion.