implement learnable skills counter alert

[nnmrts]

I could not find a way to just get all learnable skills, so I implemented that myself.

Also I check for this

G.GAME.skill_xp ~= last_skill_xp or
flat_game_skills_length ~= last_flat_game_skills_length

to not run into performance issues.

There's probably a much better way to do this, especially because this is basically polling and could probably done through functions/events, but I couldn't quite grasp which part is doing what, sorry. I tried finding points in the code where skill_xp and the skills get changed, to just call my stuff, but ran into problems with that approach.