fix make visualization nodes filled by default

[zilto]

Previously, certain nodes types didn't have a style="filled".

Setting the visualization background color to transparent with graphviz_kwargs=dict(graph_attr=dict(bgcolor="transparent")) would also make nodes transparent. It is very hard to fix by user in an ad hoc way because of graphviz's quirks.

before

after

Changes

• change default attributes
• update tests accordingly

How I tested this

• successfully ran the test suite locally

Notes

• I am making this change to help with the VSCode Hamilton extension.
• edges can be set to white or black for visibility depending on the background

Checklist

• PR has an informative and human-readable title (this will be pulled into the release notes)
• Changes are limited to a single goal (no scope creep)
• Code passed the pre-commit check & code is left cleaner/nicer than when first encountered.
• Any change in functionality is tested
• New functions are documented (with a description, list of inputs, and expected output)
• Placeholder code is flagged / future TODOs are captured in comments
• Project documentation has been updated if adding/changing functionality.

[skrawcz]

I don't think this will break anyone's stuff. So looks good.