Parse error when non-standard indentation is encountered

```python
handcrafted = [
# escaped indentation of some kind?
'fr"\\\n"',
'if _:\n _\n\\\nif _:\ _',

# mixed tabs and spaces; many variations here
'if _:\n if _:\n\t _',
]
for s in handcrafted:
assert compiles(s), repr(s)
assert not libcst_native_parses(s), repr(s)
```

_Originally posted by @Zac-HD in https://github.com/Instagram/LibCST/issues/930#issuecomment-1581965630_