feat(emacs/lean-syntax): add 'def' and 'mutual_def'
feat(emacs/lean-syntax): add 'def' and 'mutual_def'

dselsam/lean,leanprover-community/lean,johoelzl/lean,dselsam/lean,Kha/lean,soonhokong/lean-osx,rlewis1988/lean,avigad/lean,digama0/lean,leanprover-community/lean,leanprover/lean,rlewis1988/lean,Kha/lean,soonhokong/lean-osx,johoelzl/lean,rlewis1988/lean,fgdorais/lean,soonhokong/lean-windows,dselsam/lean,leanprover/lean,soonhokong/lean-osx,leodemoura/lean,digama0/lean,digama0/lean,soonhokong/lean-windows,Kha/lean,leanprover/lean,leanprover/lean,johoelzl/lean,fpvandoorn/lean,rlewis1988/lean,leanprover-community/lean,avigad/lean,leanprover-community/lean,dselsam/lean,fpvandoorn/lean,soonhokong/lean-windows,leanprover-community/lean,avigad/lean,leodemoura/lean,Kha/lean,soonhokong/lean-osx,avigad/lean,leodemoura/lean,leanprover-community/lean,rlewis1988/lean,fpvandoorn/lean,avigad/lean,johoelzl/lean,avigad/lean,fgdorais/lean,Kha/lean,rlewis1988/lean,johoelzl/lean,dselsam/lean,dselsam/lean,fgdorais/lean,fgdorais/lean,soonhokong/lean-windows,digama0/lean,fpvandoorn/lean,leodemoura/lean,soonhokong/lean-windows,fpvandoorn/lean,Kha/lean,fgdorais/lean,leanprover/lean,leodemoura/lean,digama0/lean,soonhokong/lean-osx,digama0/lean,fpvandoorn/lean,fgdorais/lean,leodemoura/lean,leanprover/lean,johoelzl/lean