Add h0 and a0 to MvuHTML and MvuAttrs libraries

[SimonJF]

The monoidal units of MvuHTML.HTML and MvuAttrs.Attr are accessed by MvuHTML.empty and MvuAttrs.empty. When creating a tree, these are quite verbose to use, so the pattern I've ended up doing is:

var a0 = MvuAttrs.empty;
var h0 = MvuHTML.empty;
div(a0, h0)

I do this so much that I think it would actually be better to have these shortcuts in the actual libraries. It's handy for me and I don't see any harm in it.

[SimonJF]

Thanks!