format: add nan_rep and null_rep kwargs

[ChristopherRussell]

Summary

Add arguments to fmt_X methods to control the string representation of nan and null values. I have written test cases but I'm not quite sure how to test the formatting result and looking for advice there.

Checklist

• I understand and agree to the Code of Conduct.
• I have followed the Style Guide for Python Code as best as possible for the submitted code.
• [] I have added pytest unit tests for any new functionality.

[machow]

Hey, thanks for opening this PR! We should be able to support the nan and null representation, via the WIP sub_missing function (#182).

This is really helpful to see--since it's clear we need to account for nan handling too.

If you have opinions on what the default representation of missing values should be, we'd love your input in the issue.

[machow]

Thanks for working on this PR -- I'm going to close for now, as the sub_missing method can handle replacing missing values. Note that it replaces both null and nan, but we should be able to support a custom substitutions by adding the more generic sub() method to GT down the road

https://posit-dev.github.io/great-tables/reference/GT.sub_missing.html#great_tables.GT.sub_missing