Set the 'colab' renderer only for Colab web notebooks

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](https://marketplace.visualstudio.com/items?itemName=Google.colab) 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/).

More context in #5471.

Author: kevineger

plotly/io/_renderers.py

@@ -488,13 +488,10 @@ def show(fig, renderer=None, validate=True, **kwargs):
 elif ipython and ipython.get_ipython():
     # Try to detect environment so that we can enable a useful
     # default renderer
-    if not default_renderer:
-        try:
-            import google.colab  # noqa: F401
 
-            default_renderer = "colab"
-        except ImportError:
-            pass
+    # Check if we're running in a Colab web notebook
+    if not default_renderer and "COLAB_NOTEBOOK_ID" in os.environ:
+        default_renderer = "colab"
 
     # Check if we're running in a Kaggle notebook
     if not default_renderer and os.path.exists("/kaggle/input"):