sensor_shell: fix assertion due to NULL pointer deref

The `SENSOR_CHAN_VSHUNT` was added in #60717 but was never
added to the `sensor_channel_name[SENSOR_CHAN_COMMON_COUNT]`
table. Since the length of `sensor_channel_name` is fixed to
`SENSOR_CHAN_COMMON_COUNT`, this means that the index at
`SENSOR_CHAN_VSHUNT` points to `NULL`. When we use the
`sensor get` command for anything bigger than
`SENSOR_CHAN_VSHUNT`, we will deref that `NULL` pointer
when we do `strcmp` in the for-loop of `parse_named_int`.

Fix this by defining `SENSOR_CHAN_VSHUNT` in the table.

Signed-off-by: Yong Cong Sin <ycsin@meta.com>