Refactor nanoplots

[machow]

This PR is for reviewing the nanoplots code, which we punted on in the name of getting it out the door!

[machow]

@rich-iannone do you mind picking this up when you get the chance?

[rich-iannone]

@rich-iannone do you mind picking this up when you get the chance?

For sure! Thanks for going over all of this :)

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 70.58824% with 10 lines in your changes are missing coverage. Please review.

:exclamation: No coverage uploaded for pull request base (main@54bffc4). Click here to learn what that means.

Files	Patch %	Lines
great_tables/_formats.py	18.18%	9 Missing :warning:
great_tables/_utils_nanoplots.py	95.65%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #246 +/- ## ======================================= Coverage ? 75.26% ======================================= Files ? 41 Lines ? 4180 Branches ? 0 ======================================= Hits ? 3146 Misses ? 1034 Partials ? 0 ```

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[machow]

@rich-iannone todo:

• Split up all the cases in the test nanoplot output into individual tests (the test names should reflect what is being tested).
• Remove xfail mark for cases where pytest.raises is being used to test something being raised.

Ping when it's ready and we can get it merged!

[machow]

LGTM--thanks for this, loving that sweet sweet test coverage!