Add __WARN__ and __DIE__ to Perlvar::is_var

Fixes test failures in t/find_matches.t

```
#   Failed test 'find_matches(__WARN__) searches for [__WARN__]...'
#   at t/find_matches.t line 45.
#          got: '__WARN__'
#     expected: '%SIG'

#   Failed test '... in perlvar'
#   at t/find_matches.t line 47.
# +----+-----+----+-------------+
# | Elt|Got  | Elt|Expected     |
# +----+-----+----+-------------+
# *   0|[]   *   0|[            *
# |    |     *   1|  'perlvar'  *
# |    |     *   2|]            *
# +----+-----+----+-------------+
# Looks like you failed 2 tests of 48.
```

%SIG doesn't contain any of the DIE or WARN variations, so it seems these all need to be added manually to %is_var.