feat: create last-modified mapping

[line-o]

Whenever this package is built in CI a module
modules/frus.xqm is added to the built XAR.

This can then be used to optimize caching in hsg-shell.

[line-o]

@joewiz pointed out that the script does only return the same date for all files. This is likely due to us only checking out a single commit in CI.
How should we proceed?

• create separate workflow that checks out several commits so that the script compiles the correct mapping
• run that script only on pushes to master - before releases, as the checkout may take a while

[line-o]

@joewiz and I found a different way to obtain the last committed date

$file='volumes%2Ffrus1981-88v01.xml'; \
curl -s "https://api.github.com/repos/HistoryAtState/frus/commits?path=$file&page=1&per_page=1" | \
jq ".[0] | .commit.author.date"

[duncdrum]

@line-o I like the idea of curling GitHub from CI. When @windauer asked me about this today, my other idea was to use artifacts on CI to keep an updated list going. Checking out the full repo is not going to be workable due to the limits of CI and the size of the repo.