Enable/use sourcemap generation from PostCSS (editor styles post-processing)

[strarsis]

What problem does this address?

Currently the added, post-processed (as for editor styles wrapper isolation) editor-styles are not source-mapped.
Inspecting the added editor styles in editor does not directly show the original styles (e.g. from frontend stylesheet).

What is your proposed solution?

Enable and use the sourcemap feature of PostCSS so that editor styles can be resolved to the original stylesheet.
As PostCSS is used for post-processing the editor styles and already supports source-maps,
this should be easy to implement.