Build wasm32-unknown-unknown in CI

[willglynn]

cargo check --wasm32-unknown-unknown is sufficient to demonstrate that the juniper and juniper_codegen crates can compile for WebAssembly, which should help avoid accidental regressions.

This is a small step towards the goals in #218.

[LegNeato]

Thanks for the PR! This looks great, but can you add it to azure pipelines as well?

[willglynn]

Added!

I'm new to Azure Pipelines and I wasn't quite sure what approach to take here, so I settled for the one that seemed least invasive. The result was a "Check WebAssembly target" step added to every job, but which runs only for the stable toolchain and is skipped everywhere else.

I also considered expanding the matrix such that "install stable + cargo check --target=wasm32-unknown-unknown" would be a separate job on all three platforms. However – as I understand it at least – this would require either a) making the regular build process conditional on whether we're building normally or checking WebAssembly, or b) adding a second unrelated job template and expanding it separately from the azure-pipelines.yml in the root. Adding a two-line step to the existing jobs and conditionally executing it seemed like the better option.

[LegNeato]

Sorry this took so long, thanks for the PR! 🍻