Custom screenshot filename

[birkirb]

Small tweak to allow for the customization of the screenshot filename through 3 additional environment variables.
Lot's of other ways to do it of course.
This one doesn't change the defaults, and overrides can be put in the .config/hypr/env.conf, much like OMARCHY_SCREENSHOT_DIR

e.g.

env = OMARCHY_SCREENSHOT_DATE_FORMAT,%Y.%m.%dT%H.%M.%S
env = OMARCHY_SCREENSHOT_PREFIX,false
env = OMARCHY_SCREENSHOT_SUFFIX,_screenshot.png