[v4] Lists: avoid float for markers?

[fvsch]

List markers are ::before pseudos aligned with float: inline-start and a negative margin.

This could theoretically create some vertical text alignment issues with custom user styles. Floats are also finicky almost by definition.

I should explore using a different technique. Some ideas:

• Negative text-indent might do the same thing but with fewer side effects.
• Theoretically the numbers/bullets and text in a list look like a grid, but using display: grid here sounds tricky and more trouble than float.

[fvsch]

I explored possible changes in #31 but it looks like the current use of float is the least bad solution.