Modernize phosh-core a bit

While working on !10 (merged) notice some things that might be nice to have but might not be as straight forward:

• phosh-core: Prefer pipewire over pulse

  It's the future so let's prep for that.

• d/control: Suggest firefox-esr-mobile-config

  This give a hint that it exists without pulling in firefox-esr itself.

• phosh-core: Suggest systemd-coredump

  We mention it all around but don't give uses any hint via the packagig system yet.

• phosh-core: Allow phosh-osk-stub as on screen keyboard

  The default is squeekbard but architectures that have trouble building Rust (or one gtk-rs) can still use the metapackge.

• phosh-core: Drop purple-lurch

  Chatty recommends it so it's pulled in by default.