fix(frontends/lean/structure_cmd): reject internal field names
fix(frontends/lean/structure_cmd): reject internal field names

Fixes #1539

leanprover-community/lean,johoelzl/lean,leanprover/lean,digama0/lean,fpvandoorn/lean,leodemoura/lean,Kha/lean,leodemoura/lean,fpvandoorn/lean,Kha/lean,leanprover/lean,leanprover-community/lean,johoelzl/lean,Kha/lean,johoelzl/lean,johoelzl/lean,avigad/lean,avigad/lean,digama0/lean,dselsam/lean,rlewis1988/lean,leanprover/lean,leodemoura/lean,fpvandoorn/lean,avigad/lean,leanprover/lean,fpvandoorn/lean,leodemoura/lean,leodemoura/lean,avigad/lean,leanprover-community/lean,leanprover-community/lean,leanprover/lean,rlewis1988/lean,digama0/lean,avigad/lean,dselsam/lean,johoelzl/lean,leodemoura/lean,fpvandoorn/lean,digama0/lean,dselsam/lean,leanprover/lean,dselsam/lean,rlewis1988/lean,Kha/lean,rlewis1988/lean,dselsam/lean,rlewis1988/lean,avigad/lean,dselsam/lean,Kha/lean,johoelzl/lean,digama0/lean,leanprover-community/lean,rlewis1988/lean,leanprover-community/lean,Kha/lean,fpvandoorn/lean,digama0/lean