Colcon Fish Extension

[threeal]

Is there any consideration for implementing an extension like colcon-bash, but for the fish shell instead?

Currently, when running inside the fish ecosystem, my workaround is to use bass to source the setup.bash script. Because of this, with a small change, it should be possible to create something like colcon-fish by slightly modifying colcon-bash.

For reference, Python virtual environments have supported the fish shell for a long time (see https://github.com/pypa/virtualenv/issues/69), so I think we should consider supporting it here as well.

[cottsay]

This seems like a natural extension.

Please consider creating an extension to do this (consider using this template package to get started). If we can work it up to sufficient quality, we can eventually move it into the colcon org.