Make ckeditor5-dev-* utils compatible with webpack 5

[pomek]

Provide a description of the task

As the migration of the entire CKEditor 5 environment is a much more complex task than we expected, and it may span a few iterations, let's update our dev utils to produce a proper build using both webpack versions: 4 and 5.

Scope (based on ckeditor/ckeditor5-dev#729):

• [utils] styles.getPostCssConfig() - the PostCSS configuration,
• [utils] Move webpack from dependencies to peerDependencies as the builds.getDllPluginWebpackConfig() function should provide a configuration without forcing using a specified version of webpack.

Related issues:

• Project migration to webpack 5 #9799
• Upgrade environment to use webpack 5 #10668

[pomek]

Move webpack from dependencies to peerDependencies as the builds.getDllPluginWebpackConfig() ...

In fact, webpack is not specified as dep/devDep: https://github.com/ckeditor/ckeditor5-dev/blob/c0ecd4fe678b797167da094a2b09700cf82be64f/packages/ckeditor5-dev-utils/package.json#L7-L36

[pomek]

I just figured out that we don't need any changes in dev-utils. Webpack 5 works fine. So, the issue is not valid.