This will enable nixpkgs -- and any other distribution that builds in a
network sandbox and/or wants to use their own corrosion package rather
than building another one -- to do so without patching taskwarrior.
Since we're already using submodules for libshared I don't think this
should make the build process any more complicated for anyone else.
See
https://github.com/NixOS/nixpkgs/issues/300679#issuecomment-2041252688
for context.
This uses CMake to build a simple Rust library (in `src/tc/rust`) that
just re-exports everything from the `taskchampion-lib` crate.
The C++ wrappers then wrap this into C++ objects with proper lifecycle
maintenance, in the `tc` namespace.
The C++ wrappers are incomplete, and missing methods are tagged with
"TODO". These will be added as needed.
