feat(frontends/lean): add 'proof-qed' notation

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(frontends/lean): add command for customizing the behavior of proof-qed blocks: we can automatically register tactics to be automatically applied before each component

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(frontends/lean): add command for customizing the behavior of proof-qed blocks: we can automatically register tactics to be automatically applied before each component