Follow-up from "enh(ci): improve cspell checker"

The following discussions from !41 (merged) should be addressed:

• @dweckmann started a discussion:

  Maybe you can use what have been done to exclude minizip for uncrustify (.sheldonignore)

• @dweckmann started a discussion:

  Shouldn't an inclusion list be easier to manage ? Unknown extension will be defacto ignored..

Also we need to improve the unit tests. For now only one case (a file that contains an error) is tested. We should test a valid file, that adding a word in a dictionary is correctly managed, etc..