Move the dictionaries into separate modules

[krassowski]

We may want to move the existing dictionaries (especially the heavier ones) into separate sub-packages e.g. jupyterlab-spellchecker-de_AT (each German dictionary weighs 4.4 MB). After #74 this becomes trivial as we only need to place the dictionary in the jupyter/dictionaries directory. We should create a helper function in jupyterlab-spellchecker that will use jupyter_packaging to place the dictionary where needed and import that function in setup.py (which will be the only file except for dictionaries) of the sub-packages.

Finally we could also create alias packages e.g. jupyterlab-spellchecker-german collecting:

• jupyterlab-spellchecker-de_AT,
• jupyterlab-spellchecker-de_CH, and
• jupyterlab-spellchecker-de_DE.

[krassowski]

Note: language packs seem be to using hyphen rather than an underscore - let's coordinate for consistent user experience.