Rename dask_labextension to dask-labextension

Would be good to rename the PyPI (and other packages) to use `-` instead of `_` in the name. This is follow up on issue ( https://github.com/conda-forge/dask_labextension-feedstock/issues/1 ).

No files or tests are named. Start by inventorying the PyPI and other package metadata for dask_labextension, then check the linked conda-forge follow-up for packaging context. Done means the relevant package names consistently use dask-labextension and the package metadata remains valid.

Written by the indexing model from the issue text.