Frontmatter Merging

When importing slides, frontmatter from main entry takes priority.

Example

Main file (slides.md):

---
src: ./cover.md
background: https://sli.dev/bar.png
class: text-center
---

Imported file (cover.md):

---
layout: cover
background: https://sli.dev/foo.png
---

# Cover

Cover Page

---
layout: cover
background: https://sli.dev/bar.png  # main entry wins
class: text-center
---

# Cover

Cover Page

Main entry > Imported file for duplicate keys.