Add an integration test using IRKernel

[blink1073]

Problem

We are currently only testing against ipykernel. If we were testing against at least one other kernel we can help prevent issues like #633.

Proposed Solution

Add a CI job that installs IRKernel in a conda environment and runs a test that starts the server, a kernel, and executes on the kernel.

Additional context

Use jupyter_kernel_test as an example for the CI setup.

[bollwyvl]

from our experiences on jupyter-lsp, conda solves with r-*+jupyter_* can be... expensive. even with mamba(forge). but especially on windows. but worth it, because there are a lot of windows r users.

it may be worth considering pre-solving and caching the solutions in ci with conda-lock... there's some skullduggery it does where this could be done in an up-front job with a cheap pip installed conda-lock, do the solves for all the platforms, and kick those down to jobs for the big three OSes.

also: it's rad someone's been working on jupyter_kernel_test ❤️‍🔥 !