feat: Adds @register_theme decorator

[dangotbanned]

Important

Prior to v5.5.0, (#3618) refactored this into alt.theme.register.
See updated examples in https://altair-viz.github.io/user_guide/customization.html#defining-a-custom-theme

Resolves one item in #3519

Fairly simple addition, but it is a nice convenience when you only use a single theme.

I also added an example that might be relevant to @joelostblom #3519 (comment)

The decorator also works well with type checkers:

[dangotbanned]

Will probably need to resolve some conflicts after merging

• feat: Adds vega-themes.json using vl_convert #3523

Edit

All good to go again

[binste]

Very useful! Thanks for adding this and sorry that you had to wait a month for a review and keep this branch in sync ;)

Only one small question regarding LiteralString, otherwise this looks good to me.

[dangotbanned]

Very useful! Thanks for adding this and sorry that you had to wait a month for a review and keep this branch in sync ;)

Thanks for getting to this @binste and no worries on the timing; this is a mini-feature after all 😄

[binste]

Good point, having autocompletion trumps type hint flexibility in this case, wasn't aware that we can't have both. Thanks for the explanation!

Would you mind just adding the link to your GitHub comment as a Python comment next to the LiteralString type hints in register_theme and in the ThemeRegistry? Else, I'll probably change the type hint in a few months when I've forgotten why LiteralString is better :)