feat: Add support for ELK layout engine and update to latest mermaid dependency

[aztechead]

In the current version of backstage-plugin-techdocs-addon-mermaid there are two issues related to unsupported layouts related to the ELK layout engine:

• Unable to use different layout for mermaid #58
• Is there a way to use elk renderer for the specific diagram? #47 (Now closed)

It looks like prior to mermaid 11.0.X the elk layout engine was included as part of the core mermaid npm dependency. That has since been moved out and now requires an additional dependency @mermaid-js/layout-elk

I went ahead and incorporated that dependency into the plugin, and registered it to the layout engine. This now supports both the legacy and modern syntax for requesting the elk renderer. This is a no-op to existing mermaid syntax that does not make use of defining a custom renderer.

Legacy

%%{init: {"flowchart": {"defaultRenderer": "elk"}} }%%

Modern

---
config:
  layout: elk
---

Screenshot

[aztechead]

@johanneswuerbach can you take a look and let me know your thoughts. It'd be really great to bring back support for the (superior) elk layout engine. 😄

[aztechead]

The GH Actions steps run locally, let me know what you think!