Standards checklist:

• The PR title is descriptive.
• The PR doesn't replicate another PR which is already open.
• I have read the contribution guide and followed all the instructions.
• The code follows the code style guide detailed in the wiki.
• The code is mine or it's from somewhere with an MIT-compatible license.
• The code is efficient, to the best of my ability, and does not waste computer resources.
• The code is stable and I have tested it myself, to the best of my abilities.
• If the code introduces new aliases, I provide a valid use case for all plugin users down below.

Changes:

• Plugin displays Spack environment and allows background theming.
• Based on the virtualenv plugin.
• Provides the function spackenv_prompt_info

Other comments:

Spack is a software/package management system commonly used in high performance computing environments.