extmod/vfs_reader: Process pending events during file reads.

Call `mp_event_handle_nowait()` in the VFS reader buffer refill path so
that pending scheduled events (USB task, network poll, etc.) get processed
during long-running import/parse/compile operations.

Without this, importing a large Python module from the filesystem blocks
for too long causing TinyUSB event queue to overflow.  For example, on
renesas-ra, running a script that imports iperf3 via mpremote run, asserts,
most likely due to SOF interrupts not getting processing:

    queue_event at lib/tinyusb/src/device/usbd.c:382
    dcd_event_handler at lib/tinyusb/src/device/usbd.c:1318
    dcd_event_sof at lib/tinyusb/src/device/dcd.h:237
    dcd_int_handler at tinyusb/src/portable/renesas/rusb2/dcd_rusb2.c:964
    <signal handler called>
    disk_ioctl at extmod/vfs_fat_diskio.c:125
    validate at lib/oofatfs/ff.c:3359
    f_read at lib/oofatfs/ff.c:3625
    file_obj_read at extmod/vfs_fat_file.c:75
    mp_stream_rw at py/stream.c:60
    mp_reader_vfs_readbyte at extmod/vfs_reader.c:59
    next_char at py/lexer.c:174
    mp_lexer_to_next at py/lexer.c:713
    mp_parse at py/parse.c:1167

Signed-off-by: iabdalkader <i.abdalkader@gmail.com>

extmod/vfs_reader.c

@@ -56,6 +56,7 @@ static mp_uint_t mp_reader_vfs_readbyte(void *data) {
             return MP_READER_EOF;
         } else {
             int errcode;
+            mp_event_handle_nowait();
             reader->buflen = mp_stream_rw(reader->file, reader->buf, reader->bufsize, &errcode, MP_STREAM_RW_READ | MP_STREAM_RW_ONCE);
             if (errcode != 0) {
                 // TODO handle errors properly