Editable install doesn't provide/link `datadir`

We currently install the Jupyter kernel spec via
```
kernel_dir = get_option('datadir') / 'share/jupyter/kernels/sagemath'
install_data('ext_data/notebook-ipython/logo.svg', install_dir: kernel_dir)
install_data('ext_data/notebook-ipython/logo-64x64.png', install_dir: kernel_dir)
kernel_data = configuration_data()
kernel_data.set('SAGE_VERSION', meson.project_version())
configure_file(
input: 'ext_data/notebook-ipython/kernel.json.in',
output: 'kernel.json',
install_dir: kernel_dir,
install: true,
configuration: kernel_data,
)
```
This works fine in a "normal" installation. But if you use an editable install, the kernelspec is not created in the python prefix and thus Jupyter cannot find the kernel.

What's the correct way to "install" such data files in editable mode?

Start with the editable-install handling for the `get_option('datadir')` path and the `ext_data/notebook-ipython` kernel files shown in the issue. Determine how editable installs should expose `kernel.json` and the two logo files so that the Jupyter kernel is created in the Python prefix and Jupyter can find it.

Written by the indexing model from the issue text.