feat(frontends/lean/server): add 'INSERT' and 'REMOVE' commands to lean 'server', make sure all commands use the same convention for numbering lines, update server.org

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(frontends/lean/server): add 'INSERT' and 'REMOVE' commands to lean 'server', make sure all commands use the same convention for numbering lines, update server.org