fix(library): remove unnecessary [fact] modifier

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
fix(frontends/lean/pp): add necessary '@' when pp.implicit is true, otherwise produced output will not even type check

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(frontends/lean/server): display current option value instead of default value

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