Re-implement `naga` development workflows using [`cargo xtask`]. Convert
`make` logic and shader test configuration as file with Bash variables
into an `xtask` crate and YAML files, respectively.
Pros:
* We now have a _portable_ workflow everywhere, which means Windows
folks and people who don't install `make` don't have to suffer.
😮💨
* Workflow logic is now relatively easy to inspect and change. Whew!
💁🏻♂️💦
* Contributors can use their existing Rust knowledge to contribute to
developer experience. 🎉
* `cargo xtask` is a relatively well-known convention for workflows in
the ecosystem.
* We can do fancy things like allow folks to run at different log levels
for workflows, depending on their tastes.
Cons:
* There's now a non-trivial compile step to project workflow.
Incremental rebuilds seem to be pretty short, though!
* Code is much more verbose than the (very) terse `make` implementation.
[`cargo xtask`]: https://github.com/matklad/cargo-xtask
