games-roguelike/cataclysm-dda: added a dev branch USE

[l29ah]

as announced in CleverRaven/Cataclysm-DDA#36949 (comment)

[leycec]

Cataclysmic. Thanks for the excellent wintertime volunteerism – as always, @l29ah.

Ordinarily, I'd prefer to unconditionally switch the EGIT_BRANCH from master to dev rather than add another USE flag. That said, @ZhilkinSerg's commentary does suggest that the new dev branch might just be a temporary placeholder that will go away after the 0.E release. If so, then adding another USE flag is actually the Right Thing To Do.™

In short, time will tell. I'm delighted to merge this as is and grateful for your keen eye. Let's keep an eye on upstream, though. If unstable feature development ends up permanently moving over to the new dev branch, we'll probably want to:

1. Revert this commit.
2. Unconditionally switch the EGIT_BRANCH from master to dev.

Thanks again, @l29ah. You're awesomeness!

[KorGgenT]

as a developer of C:DDA i'll confirm that it is temporary and the dev branch will be merged to master after the 0.E stable release

[ZhilkinSerg]

FYI: We started to make hotfix releases for stable versions:

• https://github.com/CleverRaven/Cataclysm-DDA/releases/tag/0.E-1
• https://github.com/CleverRaven/Cataclysm-DDA/releases/tag/0.E-2

It is not necessary that each and every stable release in the future will have such hotfix, but it is highly likely.