Add the system_fonts() helper function

[rich-iannone]

This introduces the system_fonts() helper function, and this allows for obtaining a list of themed fonts, the collection of which is expected to work well across computing platforms. There are 15 font stacks in total, and the function is useful within tab_options() (for the table_font_names parameter).

Fixes: #157

[machow]

I left a comment on improving font stacks code

[machow]

Can we make this a dictionary? Something like...

FONT_STACKS = {
    "system-ui": ["system-ui", "sans-serif"],
    "transitional": ...
}

Then system fonts could return something like FONT_STACKS[name] (with handling for emojis, and erroring on bad font stack name.

It might be worth putting FONT_STACKS its own file (similar to _data_color/constants.py), or maybe isn't too bad at the top of _helpers.py

[rich-iannone]

Will merge this even in it's early state (agreement from pairing). But will revisit again.