Support "include" option in config file

[emersonsiega]

Motivation
We have multiple apps (micro apps) using almost the same build.yaml file.
It would be nice if we can have a base file to be used as reference.

lints package have this feature. It's a nice usage example :)

include: package:my_local_package/slang_settings.yaml

targets:
  $default:
    builders:
      slang_build_runner:
          # new settings or overrides

[Tienisto]

Hi, build.yaml is a feature by build_runner. I don't know if they support it