Formatter: don't break single-argument multi-line strings if the first line "fits"

[charliermarsh]

See: https://github.com/psf/black/pull/1879/files.

In Preview, Black now reformats:

textwrap.dedent(
    """
    This is a
    multiline string
"""
)

to:

textwrap.dedent("""
    This is a
    multiline string
""")

https://black.vercel.app/?version=main&state=_Td6WFoAAATm1rRGAgAhARYAAAB0L-Wj4AGlAIldAD2IimZxl1N_WmhEpla9DGFkUapmMYaVDChQFFzFY8lTc9V-psgZJEaxBwinapUZzdJvX8NXjM1PIFkv5tdgxiTStTWHDKc-RWzWjZvcHL8-afLVYsHq6y_3a6xN_lELxXHP-HR-0jzuepYl65gRWE-um2cLzTZWAycQpu0Bz_5igXnUT4SeJh8AAAAAAHz0sc5o1W6jAAGlAaYDAACwhaLdscRn-wIAAAAABFla

[charliermarsh]

(This is part of preview style, so perhaps not required for now.)

[charliermarsh]

@MichaReiser -- do we have a general opinion on preview style? My assumptions:

• If we implement formatting that does match preview style (but doesn't match Black's stable formatting), that's okay.
• However, preview style is currently a nice-to-have and not required to ship.

[MichaReiser]

Preview style isn't a priority right now. We can implement it but we need to add a preview option first and gate the logic accordingly

[MichaReiser]

Tracked as part of #6935