Properties feature: add space after inserting new YAML frontmatter

When adding Properties using the new feature in 1.4.0, if the note does not have YAML frontmatter yet, the frontmatter is generated without any space after the bottom `---`.

This feels a bit cramped:

---
tags:
  - foo
---
# The rest of the note here...

Blah blah blah

Better:

---
tags:
  - foo
---

# The rest of the note here...

Blah blah blah
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