Mark expressions with `from()` as safe

[RubenVerborgh]

Currently, all expressions with function calls are considered unsafe.

We might want to be more permissive and consider things like [.from('https://ruben.verborgh.org/profile/')['https://data.verborgh.org/people/joachim_van_herwegen'].firstName]https://solid.github.io/ldflex-playground/#.from('https%3A%2F%2Fruben.verborgh.org%2Fprofile%2F')%5B'https%3A%2F%2Fdata.verborgh.org%2Fpeople%2Fjoachim_van_herwegen'%5D.firstName safe, such that they can be loaded automatically, and have an associated SPARQL query.