[Feature] Support badges of latest version on your package server (`pkg.pkl-lang.org`)

I want to add a badge to my README file that shows the latest version that consumers can use.
Similar to other badges supported by https://shields.io

I guess a Pkl server requires to send the latest version if requested with `latest`or something so shields.io (or others) can use that API to display the latest version.

Its a bit related/similar to https://github.com/apple/pkl/issues/278 or better [to this comment](https://github.com/apple/pkl/issues/278#issuecomment-1977152027).

If you support using e.g. `package://pkg.pkl-lang.org/github.com/stefma/pkl-gha/com.github.action@latest` and this would redirect to the latest version (0.0.1 🤷), then it would be possible to implement... I guess 😁

Also not sure if this issue should be better placed at https://github.com/apple/pkl-lang.org 🤷
Feel free to move it.

Start by reviewing issue 278 and its linked comment, then inspect the package-server context at pkg.pkl-lang.org and the possible pkl-lang.org repository. Clarify which server should own the change and define the API or package-resolution behavior that Shields.io could consume. Done means an agreed implementation scope and support for displaying the latest usable package version.

Written by the indexing model from the issue text.