Add our own variable names into the allowed words dictionary

If you include --dot in your CSpell options (as we document here) then I have found that some our own Gitlab Templates variables cause spelling errors, eg 'FLAGWORDS' in this downstream cspell job

Gitlab Templates has solved this by adding these names to our own project dictionary. But I would consider it to be a bug if you try to use any documented variable in your own pipeline and it immediately fails spelling. If we are providing these variables for use, then I suggest we should add the words into the allowed list for all Contrib.

This can be done very easily in prepare-cspell.php as we already do this kind of thing