Pickling widgets for archival

[Vtec234]

A pickled widget is a widget module together with its Props serialized for the purpose of archival, for example on a static webpage via LeanInk.

We should provide a mechanism to pickle widgets, and a stub of the @leanprover/infoview module which pickled widgets would import. Some things obviously cannot work when there is no Lean server present, for example attempting an RPC call in the stub environment will fail, but as long as the widget Props contain sufficient data to display something informative, they can be archived.