chore: copy sourcemaps to public/static/

Because even though we're not linking to them, it appears that the
profiler (dd-trace-js) is going to look for them locally by virtue of
its dependency on this `SourceMapper` class:

- https://github.com/DataDog/pprof-nodejs/blob/012af7560c588d6604b40a86c5c80aad39d230d0/ts/src/sourcemapper/sourcemapper.ts#L152

webpack.production.config.js

@@ -87,6 +87,9 @@ module.exports = BUILDS.map(({name, filename, targets, terserOptions}) => {
                 const src = path.resolve(__dirname, 'dist', asset);
                 const dest = path.resolve(__dirname, 'public', 'static', asset);
                 fse.copySync(src, dest);
+                if (fs.existsSync(src + '.map')) {
+                  fse.copySync(src + '.map', dest + '.map');
+                }
                 if (fs.existsSync(src + '.LICENSE.txt')) {
                   fse.copySync(src + '.LICENSE.txt', dest + '.LICENSE.txt');
                 }