Add spell check github action

[Nightblade]

Current state: proof of concept / testing / review.

Questions / suggestions / comments welcome.

• Automatically run cspell-action on pull-requests made to dev.
• Trigger manually to fully check a branch/tag/SHA.
• Use cspell config/dictionary files from a secondary repo (temporarily nightblade/pob-dict).

I've been regularly running cspell locally on the dev branch and at the moment it reports zero spelling issues. Keen observers will notice there are still ... a few. These should only be pre-existing errors that are yet to be addressed, GGG errors, or idiomatic PoE/PoB words.

Further detail available upon request.