Issue Description:

Today the build system and other subsystems send some output directly to a stdout stream (such as output printed by plugins), while diagnostics go through the observability system.

It would be better to send everything through the same funnel, which would allow:

• proper ordering between emitted diagnostics and plain-text output

• a way to add metadata to the output (such as identifying which plugin emitted the output)

A good approach would probably be to split each line of output into a separately emittable diagnostic.

The presentation of that output is then a separate concern, but its logic would be simpler if everything went through the same API and had metadata associated with it.

A suggestion would be to have a new `output` severity between `info` and `warning` which would be shown by default (to preserve current behavior of emitting stdout stream text).