feat(frontends/lean/scanner): allow upper-case greek letters in identifiers but Pi and Sigma
feat(frontends/lean/scanner): allow upper-case greek letters in identifiers but Pi and Sigma

eigengrau/lean,soonhokong/lean-windows,javra/lean,leanprover-community/lean,htzh/lean,johoelzl/lean,rlewis1988/lean,eigengrau/lean,soonhokong/lean-windows,javra/lean,UlrikBuchholtz/lean,fpvandoorn/lean2,c-cube/lean,fgdorais/lean,c-cube/lean,fgdorais/lean,javra/lean,fpvandoorn/lean,c-cube/lean,eigengrau/lean,avigad/lean,avigad/lean,Kha/lean,htzh/lean,javra/lean,leanprover-community/lean,leanprover-community/lean,UlrikBuchholtz/lean,johoelzl/lean,leanprover-community/lean,sp3ctum/lean,eigengrau/lean,fpvandoorn/lean2,soonhokong/lean,sp3ctum/lean,fpvandoorn/lean2,Kha/lean,leanprover/lean,rlewis1988/lean,dselsam/lean,UlrikBuchholtz/lean,digama0/lean,Kha/lean,leodemoura/lean,leodemoura/lean,soonhokong/lean-windows,Kha/lean,leanprover/lean,soonhokong/lean,dselsam/lean,sp3ctum/lean,fpvandoorn/lean2,rlewis1988/lean,leanprover/lean,soonhokong/lean-osx,rlewis1988/lean,fgdorais/lean,leanprover/lean,soonhokong/lean,fpvandoorn/lean,digama0/lean,c-cube/lean,dselsam/lean,soonhokong/lean-osx,avigad/lean,Kha/lean,leodemoura/lean,leodemoura/lean,digama0/lean,Kha/lean,rlewis1988/lean,eigengrau/lean,fpvandoorn/lean,leanprover/lean,digama0/lean,johoelzl/lean,johoelzl/lean,soonhokong/lean-osx,UlrikBuchholtz/lean,avigad/lean,soonhokong/lean-windows,htzh/lean,soonhokong/lean,digama0/lean,leanprover-community/lean,fgdorais/lean,levnach/lean,fpvandoorn/lean2,rlewis1988/lean,fpvandoorn/lean,fpvandoorn/lean,dselsam/lean,levnach/lean,soonhokong/lean-osx,c-cube/lean,leodemoura/lean,sp3ctum/lean,levnach/lean,soonhokong/lean-windows,leodemoura/lean,UlrikBuchholtz/lean,htzh/lean,levnach/lean,leanprover/lean,fpvandoorn/lean,johoelzl/lean,dselsam/lean,johoelzl/lean,javra/lean,leanprover-community/lean,avigad/lean,avigad/lean,soonhokong/lean,levnach/lean,digama0/lean,htzh/lean,fgdorais/lean,soonhokong/lean-osx,fgdorais/lean,dselsam/lean,sp3ctum/lean