remove renderfile property from notcurses_option

[dankamongmen]

renderfile was added very early in Notcurses development, probably in 2019, and I've never used it once. It's time for it to go. Don't break ABI by pulling it out yet, but mark it as deprecated in the header file and docs, and check for its presence. If it is defined, ignore it and print a diagnostic. Remove it in ABI3.

[dankamongmen]

in @igo95862 's wrappers, we have:

    if (NULL != render_fd_object)                                                                                            
    {                                                                                                                        
        long render_fd = GNU_PY_LONG_CHECK(render_fd_object);                                                                
                                                                                                                             
        FILE *renderfp = fdopen((int)render_fd, "w");                                                                        
        if (NULL == renderfp)                                                                                                
        {                                                                                                                    
            PyErr_SetString(PyExc_ValueError, "Failed to open render file descriptor.");                                     
            return NULL;                                                                                                     
        }                                                                                                                    
                                                                                                                             
        options.renderfp = renderfp;                                                                                         
    }           

it looks like this is setting up the renderfp option, but i'm not sure exactly what render_fd_object does, hrmmm. ok, it's coming in from the kwds tuples. so i guess we just want to remove that keypair?

[dankamongmen]

alright. removed it from C implementation, new python, and rust.

[dankamongmen]

i've added a note to remove the field for ABI3 in #1777.