ReMarkdown v4

[fvsch]

I'm planning a few maintenance updates, which are technically breaking changes:

1. Update Sass library to work with @use instead of @import. It's a breaking change for people who use Sass to build their own version of ReMarkdown.
2. Use more recent CSS features when possible, targeting either "Baseline: Widely Available" or "Baseline: Newly Available".

So that's going to be ReMarkdown v4. If I think of new features I want to add, I might throw those in, but for now nothing comes to mind.

Potential features

• Scope styles with @scope (…) to (…) (MDN).