This directory is generated instead of the x86_64-unknown-linux-gnu
directory when building on an ARM64/AArch64 Mac, so this change adds
that directory to the gitignore config.
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.