Set the 'colab' renderer only for Colab web notebooks

[kevineger]

Full context in #5471.

The current implementation detects if the google.colab module is available. This has a couple problems:

1. Anyone can install that library, outside of Colab.
2. The recently launched Colab VS Code extension does not work with that web-based renderer.

Snapping to the conventions used for the other notebooks/renderers (e.g. Kaggle, Azure), this change now looks for the presence of the COLAB_NOTEBOOK_ID environment variable. I'm an engineer on Colab and have verified that we set this on kernel init for all notebooks accessed through our web app (https://colab.research.google.com/).

Code PR

• I have read through the contributing notes and understand the structure of the package. In particular, if my PR modifies code of plotly.graph_objects, my modifications concern the code generator and not the generated files.
• I have added tests or modified existing tests.
  • There were no existing tests for this that I could find.
• For a new feature, I have added documentation examples (please see the doc checklist as well).
  • Not a new feature.
• I have added a CHANGELOG entry if changing anything substantial.
  • Not substantial.
• For a new feature or a change in behavior, I have updated the relevant docstrings in the code.
  • Not a substantial feature change. I can update any docs if requested.