Support wgsl diagnostic directives.

Wgpu added this in v24, but I didn't figure out how to get them working properly before we merged that PR so they're currently unsupported. There's a basic test for them written. [The test](https://github.com/bevyengine/naga_oil/blob/f8e7d536b2fc9b3170f5579594da0aa9b332cd71/src/compose/test.rs#L1267) and [the test input](https://github.com/bevyengine/naga_oil/tree/master/src/compose/tests/diagnostic_filters) (though the expected output [here](https://github.com/bevyengine/naga_oil/blob/master/src/compose/tests/expected/diagnostic_filters.txt) is incorrect). The test currently panics due to the `unwrap` because the shader module creation is erroring for some reason.