Fix stylist indentation with a formfeed (#7489)
**Summary** In python, a formfeed is technically undefined behaviour
(https://docs.python.org/3/reference/lexical_analysis.html#indentation):
> A formfeed character may be present at the start of the line; it will
be ignored for
> the indentation calculations above. Formfeed characters occurring
elsewhere in the
> leading whitespace have an undefined effect (for instance, they may
reset the space
> count to zero).
In practice, they just reset the indentation:
https://github.com/python/cpython/blob/df8b3a46a7aa369f246a09ffd11ceedf1d34e921/Parser/tokenizer.c#L1819-L1821
https://github.com/astral-sh/ruff/blob/a41bb2733fe75a71f4cf6d4bb21e659fc4630b30/crates/ruff_python_parser/src/lexer.rs#L664-L667
The stylist didn't handle formfeeds previously and would produce invalid
indents. The remedy is to cut everything before a form feed.
Checks box for
https://github.com/astral-sh/ruff/issues/7455#issuecomment-1722458825
**Test Plan** Unit test for the stylist and a regression test for the
rule
