Flag indicating the use of a new version of Bootstrap for developers

[kpodemski]

We should add a flag to theme.yml and to the core, allowing the developer to check whether the theme is based on Bootstrap5. It is to distinguish the use of Classic vs. Hummingbird as a base of the theme. Why? Because it impacts front-end work from those developers.

I'm pretty sure I discussed it with @MatShir and @Hlavtox as an absolute must-have, but I cannot find an issue about it.