add extension point for custom link protocol renderers in markdown (#1639)

This PR allows for custom link protocols to be declared and rendered in markdown.
A new extension point markdown-renderer.link.protocol allows for renderers to hook into the api and implement any custom protocol.

Example:

[description](myprotocol:somelink)
binder.bind("markdown-renderer.link.protocol", { protocol: "myprotocol", renderer: MyProtocolRenderer })
This renderer functions similar to link renderers and receives the href and the description. The latter as the children property.

This PR also fixes two bugs where external- and anchor links were not correctly rendered in pull requests by the review-plugin.

Co-authored-by: Eduard Heimbuch <eduard.heimbuch@cloudogu.com>

docs/en/development/plugins/extension-points.md

@@ -62,6 +62,17 @@ The following extension points are provided for the frontend:
 - Dynamic extension point for custom language-specific renderers
 - Overrides the default Syntax Highlighter
 - Used by the Markdown Plantuml Plugin
+### markdown-renderer.link.protocol
+- Define custom protocols and their renderers for links in markdown
+
+Example:
+```markdown
+[description](myprotocol:somelink)
+```
+
+```typescript
+binder.bind("markdown-renderer.link.protocol", { protocol: "myprotocol", renderer: MyProtocolRenderer })
+```
 
 # Deprecated