Allow providing additional conda envs to update in topgrade.toml

[maxkapur]

What does this PR do

This PR adds a [conda] block to topgrade.toml with two optional lists env_names and env_paths. These lists specify additional conda environments whose packages are updated using conda env update -n env_name or conda env update -p env_path, respectively.

I modified run_conda_update() to update the base conda environment first, then additional named environments and environment paths.

Standards checklist

• The PR title is descriptive
• I have read CONTRIBUTING.md
• Optional: I have tested the code myself
• n/a: If this PR introduces new user-facing messages they are translated

For new steps

• Optional: Topgrade skips this step where needed
• Optional: The --dry-run option works with this step
• Optional: The --yes option works with this step if it is supported by
  the underlying command

If you developed a feature or a bug fix for someone else and you do not have the
means to test it, please tag this person here.

[maxkapur]

Leaving this as a draft PR as it will require conflict resolution if/when #1047 is merged, but happy to hear thoughts/feedback.