Cross-repository discussion references

[marcastel]

Feature request
Provide syntax and support for discussion URL shortcodes in Markdown text, commits, Pas, etc.

Description
Issue shortcodes are great -- cli/cli#1 becomes cli/cli#1 (and is way shorter than https://github.com/cli/cli/pull/1).

An equivalent shotcode for https://github.com/cli/cli/discussions/3242 would be great.