Fix stylist indentation with a formfeed

[konstin]

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

ruff/crates/ruff_python_parser/src/lexer.rs

Lines 664 to 667 in a41bb27

	'\x0C' => {
	self.cursor.bump();
	indentation = Indentation::root();
	}

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 #7455 (comment)

Test Plan Unit test for the stylist and a regression test for the rule

[github-actions]

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