Spike: Mermaid plugin

[as-op]

Passion project spike

follow up to #91

see opf/openproject#25874 for integration into OpenProject

• add an up-to-date mermaid plugin to the build
  • we cannot us the experimental https://github.com/ckeditor/ckeditor5-mermaid , it does not work properly with our build setup and includes a mermaid.js dependency which should be loaded only once by the OP application
  • vendored a fork at https://github.com/TriliumNext/Trilium/tree/main/packages/ckeditor5/src/plugins/mermaid
  • added an allowlist of supported graphs and a dropdown select into the editor