Adds a new `comm/rust` directory used to build the upstream `gkrust-shared`
library within the newly provided `gkrust` Rust library.
Expose a new dedicated workspace and vendoring system, allowing for
Thunderbird-only dependencies within the tree, and solve the issue of having
drift in Cargo.lock between Thunderbird and Firefox.
New mach commands for managing the workflow around Rust with Thunderbird:
./mach tb-rust sync
./mach tb-rust vendor
Differential Revision: https://phabricator.services.mozilla.com/D193543
--HG--
extra : amend_source : ca6dc59ef386f4c62f53e79dac8cdb6caef8104e
e906c84d0d