Add BSContainer layout

[mrlem]

I've noticed you've been implementing components, I figured Bootstrap layouts might come in handy as well (though kobweb has some implementations for Columns and such, but not really equivalent to what BS offers).

So for this first layout, I picked Container which is fairly simple, yet widely used.
I tried to be in line with the code-style of the project.

Implementation choices

• I decided to create a dedicated Breakpoint enum, as it is a framework-wide notion. This is of course debatable.
• Doing this, I realized there was an unused OffcanvasBreakpoint class. I don't know if it was forgotten or if you have plans with it, so I left it alone (though I feel it could benefit from the newly created Breakpoint class).

Scope

This PR only includes the basic Container layout, not the Grid layout that can be added on it.