Add divider as block type

[virgile-dev]

Feature Request

Is your feature request related to a problem or unsupported use case? Please describe.
Some users want to use line as dividers in their doc.

Describe the solution you'd like
Add a block type that insert a line in the document.
A user can add a line using the block type selector (+ or /) or by entering --- or ===

Discovery, Documentation, Adoption, Migration Strategy

• See if BlockNotejs would be interesting in having it as standard block or if we should build a custom block