[Draft] Add $enable-host to generate a different bootstrap.css for web components

[julien-deramond]

Warning
Heavily draft

Another possibility to tackle #36688 (Closes #36688)

The principle here is to offer a $enable-host Scss var (false by default). This var is associated with a root mixin allowing to switch between :root and :host:

• :root when $enable-host is false (default)
• :host when $enable-host is true

I've added some unit tests to check this behavior.

It differs from #37162 which suggests to use :root, :host that is not really useful for all folks and projects.

The disadvantage here is that it's only available via Sass compilation. But maybe we can consider that this usage with web components is an "expert mode".
We also talked about a bootstrap.*.host.*.css with @vprothais but I'm a bit afraid of what it could imply in combination with the regular bootstrap.*.css* files

• Handle site/content/docs/5.3/customize/css-variables.md modifications (if any needed)
• Test this new file (with $enable-host set to true) in different use cases amongst Use :host in selectors to better support web components #36688 (comment)
• Add $enable-host in the documentation and explain the impacts for web components

Thoughts??

[lominming]

This would be super useful for my use-case. I am trying to figure out the best way to tackle this right now. Ideally I have a way to hoist all the variables out to :host.