This PR adds a flag to Next.js to enable Webpack options to improve
memory usage. See https://github.com/webpack/webpack-sources/pull/155
for a full description of the changes and impact on memory.
This PR adds a patch to `webpack-sources` temporarily that contains the
fixes as the real changes are iterated on to merge upstream in the
`webpack/webpack-sources` repository. After that is done, the patch will
be reverted and the latest `webpack-sources` version will be updated in
Next.js.
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