Migrate to webpack-blocks v1

Webpack-block v1 is a release candidate and the API for blocks has changed a bit, see https://github.com/andywer/webpack-blocks/blob/master/docs/BLOCK-CREATION.md

I believe your block should now be:
```js
function serverSourceMap(devtool = 'source-map') {
return (context, { addPlugin }) => {
return (prevConfig) => {
return Object.assign(
addPlugin(new webpack.BannerPlugin({
banner: 'require("source-map-support").install();',
raw: true,
entryOnly: false,
}))(prevConfig),
{ devtool }
);
};
};
}
```

I was too lazy to write a PR, but let me know if you'd be interested.