Fix type hint of @v_args() decorator
Before, the test script below fails Pyright's type-checking:
```
error: "assert_type" mismatch: expected "str" but received "Any | Unknown" (reportAssertTypeFailure)
```
(VSCode's python extension, Pylance, uses Pyright internally)
Test script:
```python
import sys
from lark import Transformer, Token, Tree, v_args
if sys.version_info < (3, 11):
from typing_extensions import assert_type, reveal_type
else:
from typing import assert_type, reveal_type
@v_args()
class Foo(Transformer[Token, str]):
def __init__(self):
super().__init__()
def foo(self, meta, args):
return 'baz'
bar = Foo().transform(Tree('foo', []))
assert_type(bar, str)
```
I've not added this test because it would require adding pyright as a
second typechecker dependency when the project already uses mypy
fix: #1579