feat: arrange slides on a grid - #2746

Adds two-dimensional slide navigation: ---- (or nested: true in a slide's metadata block) places that slide/slides below the preceding slide. Arrow keys navigate the grid, and the overview reflects its layout. Resolves #1187. Also tracked in #1515.

Example is in the PR too, but for brevity:

---
title: Topic One
---

This is our first major topic today

----

This is now a nested slide under the first topic

----

Still nested until we get back to another three-dash

---
title: Topic Two
---

This is the second topic of the day

---
nested: true
---

This is now a nested slide under the second topic, manually using the metadata tag

---
title: Topic Three
---

This is our third major topic today

---
src: ./topic-three.md
nested: true
---

<!-- This whole subdeck becomes slide 2-n of the col for topic 3 -->

Grid navigation activates automatically when the presentation contains nested slides. Existing presentations using --- retain their current layout. This does however change the meaning ---- if used in existing presentations, which previously were treated as regular slide breaks. I'd be open to requiring a presentation-level opt-in flag if that compatibility concern warrants it. I'll also note that if migrating from slidev with minimal changes is desired, we should use -- instead of ----, but that requires parser changes.