Validate package repo link exists

[ntjess]

Description

Currently, packages can specify their repository in typst.toml, but this link is unvalidated (see Amelia-Mowers/typst-tabut#12 for an example)

It would be nice if one of the PR checks confirmed that URL-like fields in the config return valid status codes. As a bonus, if a link is broken, it would help maintainers notice this quickly if there was a visual indication on the packages webpage (e.g. a red slash through the link icon)

[laurmaedje]

What would be needed for this is some sort of ephemeral check. Right now, full CI runs for all packages on every PR, so if a link was okay, but then became broken, CI would keep failing with no way to fix it.

[slashformotion]

Maybe a solution would be to detect which packages are being updated/changed and then only check if the homepage of thoses package is till available.