refactor(theme-preset): Streamline S3 classes, store theme info in sass variables

[gadenbuie]

This PR improves our use of S3 classes around preset themes:

1. bs_preset_bundle() takes a preset object but doesn't use S3 for dispatching. If it's not a preset or if preset$type is missing, the bundle returns NULL.

2. bs_theme(), when a preset is added to the theme, returns an object with class bs_theme_with_preset, in addition to a version-specific class. To determine the preset name and type, call theme_preset_info() to read the values from the sass bundle.

3. The preset bundles now include two SASS variables, $bslib-preset-name and $bslib-prest-type, that encode the name and type of the preset. We already included $bootstrap-version, so it's now possible to completely extract the theme information from the SASS bundles. I added theme_preset_info() that does just this.

   • Extracting variables from the SASS layers is slow enough that I think it's worth testing for the presence of bs_version_N or bs_theme_with_preset classes before making the more expensive call to bs_get_variables().
   • The SASS variables are now defined in bs_theme_init() so they're guaranteed to exist for all bs_theme()-created themes. I've also added rules that create CSS variables, e.g. --bslib-bootstrap-version and --bslib-prest-name etc, to allow us to know the Bootstrap version and our preset values even in the compiled CSS.

[cpsievert]

I still doesn't feel quite right that $theme_class exists. Could we get rid of it, and do something like this?

Suggested change

	if (!is.null(preset$theme_class)) {
	bundle <- add_class(bundle, preset$theme_class)
	}
	if (!is.null(preset$type)) {
	bundle <- add_class(bundle, theme_preset_class())
	}

[gadenbuie]

I don't mind, I made the change to a constant in 3c44cc3.

The reason I didn't originally was because it felt like preset$type being NULL was an indirect signal of whether this class should be added. I included preset$theme_class so that if we changed that assumption later we'd only need to reason about the preset object.

But the problem is that we have to remove preset the preset class as part of bs_theme_update() ... so I'm on board with constant as a the signal that we're doing something special.