feat(frontends/lean): display warning message when importing file that uses 'sorry'

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(frontends/lean/scanner): allow letter-like unicode characters and sub/super-scripts in identifiers

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
checkpoint

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>