Add ability to check diff without JS runtime

[fuelen]

I tried to ask Claude to update my dependencies by asking it to run mix hex.outdated and checking the diff URL at the bottom of the output, read the changelog, etc. It wasn't able to do so because the diff is dynamically loaded. It would be great to have a URL like

https://diff.hex.pm/diff/<package_name>/<version1>..<version2>.patch

[ericmj]

We have mix hex.package diff PACKAGE VERSION1..VERSION2 maybe we should add a hint to the end of mix hex.outdated so agents can catch it?

[fuelen]

Oh, I didn't know about mix hex.package diff.
I think it makes sense, since hex.outdated already has a section related to diff and it is a good fit

[ericmj]

Closing in favor of: hexpm/hex#1137