Add postfix snippet for `unpack`

[ofseed]

Sometimes we are not sure whether functions return a tuple or a table, assuming we need to get a position containing row and col, wrote:

local position = get_position()

Then we found position here is an integer, not a table (by inlay hint or diagnostic info), which means get position returns a tuple. In this situation, a postfix snippet will be useful because our cursor is at the end of the line. By simply typing @unpack we can turn it into a tuple.

local position = unpack(get_position())

That's why I think this snippet is useful.

[sumneko]

Thank you!