feat(frontends/lean): add '[protected]' modifier

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
refactor(library): use '[protected]' modifier

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(emacs/lean-syntax): add syntax-highlight for declaration modifiers

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