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>

scm-ui/ui-components/src/index.ts

@@ -95,6 +95,7 @@ export * from "./repos";
 export * from "./table";
 export * from "./toast";
 export * from "./popover";
+export * from "./markdown/markdownExtensions";
 
 export {
   File,