Decouple core from side-effects

[roberto-red]

Description

@penpot-export/core currently logs to the console and write files, without any way to control that behaviour for the consumer.

This may be undesired for some use-cases. E.g. someone that wants to hook into the file before it's written to use PostCSS or CSSTree or apply any transformation.

Expected output

Move logging to the CLI. This could be made injecting a logger function. But I'll just prefer to use a generator function in core and use message passing with the CLI, then let the CLI log the way it wants.

Return file contents instead of writing files directly. Expose a function to write files, if the logic happens to be non-trivial. When written as a generator function, each file will be another message yielded. The file path configuration won't be required.

Opportunity

Using a generator function will allow the core transformation to be treated as a thread, and a consumer (like the CLI) will gain great abilities like pause and resume execution at will. This may be used to make the generation interactive or quiet, or expose some user hooks.