Add a C++ wrapper around TC FFI

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.

test/.gitignore

@@ -34,6 +34,7 @@ variant_partial.t
 variant_subtract.t
 variant_xor.t
 view.t
+tc.t
 tw-2689.t
 
 json_test