Fix and improve copy keybind

[WaitingIdly]

changes in this PR:

• add a check that mc.player != null for the copy key (otherwise you could crash if pressing it while loading into a world).
• allow running on servers, not just singleplayer.
• fix using a private field to get the slot because it appeared public in the dev env due to ATs from other mods.
• fix running on screens where the keyboard is being consumed (ie jei search bar).
• fix jei compat running on every screen, not just screens that jei is displayed on.
• add shorthand method copyFromPlayer to combine significant duplicate logic.
• fix copyFromPos using BlockPos instead of the ray trace directly.