feat: Add edge labels

[georgefst]

Behaviour is probably what we want, but implementation still a little messy.

We only display labels for let nodes for now. Eventually we might want to do more with this, including even labelling every single edge in beginner mode.

I had previously thought that labels should be on the outgoing connection rather than the edge itself, out of concern that the latter would look a bit like a node, and as an indication that the number of edges and their names is a property of the parent. But this design, with borderless labels, as iterated on with @dhess last week, actually looks alright.

Closes #24, I suppose.

[dhess]

This is rebased on main and seems to be working. Here's what I'd still like to do before merging:

• Fix the typeface for the labels; it should be the same as our code font.
• Tweak the styling of the translucent background a bit.
• Lengthen the edges, at least for edges that have labels.
• Investigate the TODO why? this seems like a bad abstraction comment.

I suspect that lengthening the edges might be tricky, so it might make more sense to punt that until later.

[dhess]

Screenshot of what this looks like at the moment:

[dhess]

These edge labels don't play nicely with one of our node label styles; e.g.:

It might be time to ditch the overlay-style labels and move to inline labels.

[dhess]

Upon further review, I think that rendering the edge labels identically to how we render their corresponding syntax nodes makes sense... at least for lets:

[dhess]

The code in this commit is maybe a bit rough, but I think it's good enough, and I don't want to hold up this feature any longer.

I'm pretty happy with rendering the edge labels as solid-filled "roundels," so I'm keeping that change.

I'm going to pass on the edge-lengthening bit for now.

[dhess]

Ugh, turns out this branch did not include the roundel-style edge labels. I'll commit that change shortly.