Integrate the "place actions" API into a Desktop Firefox build

[rfk]

Once the API from #3 exists, we'll need to figure out how to expose it to Desktop Firefox. I expect there are some existing patterns for doing this from other rust-based Firefox components, but I don't know enough about them to write them down in this bug.

Idly pondering this after dinner, we might be able to use MozURL for inspiration. It's a wrapper around the Rust url crate. Looks like we have a rust-url-capi crate that translates the idiomatic url API into a C-like interface, which is then called from a C++ wrapper class. In our case, the C++ wrapper class could implement a ChromeOnly WebIDL interface, and we'd write some glue code to call into our library.