This will set the `mode` of Webpack to `production` for release builds.
It will also stop inlining the sourcemap and instead produce a separate
file which is excluded by `.vscodeignore`.
In terms of the bundled extension, this will add 1 file
(`out/webview.js.LICENSE.txt`). It decreases the size of the VSIX file
from 4.28MB to 1.77MB.
