Merge multiple environment files

[liamhuber]

• Modify condamerge.py to accept an arbitrary number of file input to concatenate.
• Make a new action for merging environments
• Modify the other custom actions so that additional environments for docs and notebooks (and perhaps other stuff) gets added straight from the .support folder here, i.e. .support/environment-docs.yml with nbsphinx in it. (Still allow for downstream users to add one or more additional files. Multiple should be possible with multiline input)

[liamhuber]

I've been pushing straight to main since this is all brand new anyhow, and I don't want to track down all the commits, but this got done.