Update to Lean 4

[rommeswi]

I was wondering whether you plan to update your layer to lean 4?

lean4-mode is still not on elpa but can be loaded via recipe

Currently, there is a messy transition situation that makes lean almost completely inaccessible in Emacs for the average user. Making your layer usable both with Lean 3 and Lean 4 would help at least the Spacemacs users a lot.

There are also some suggestions for evil keybindings for lean4-mode. These would make sense in the layer config (but probably not in lean4-mode itself as suggested by the cited issue).