epic: Implement nano linechart

[machow]

trafficstars

https://gt.rstudio.com/reference/cols_nanoplot.html

[machow]

From pairing w/ @rich-iannone, let's make the nanoplot a formatter for now, and code it up similar to great_tables._formats.FmtImage. Basically, a dataclass with a to_html() method. That way, we can test it out, fit it in to our architecture, before rolling .cols_add().