Combine f-strings which would be placed on the same line

[pushfoo]

Describe the style change

Combine short f-string segements which would be placed on the same line

Examples in the current Black style

1. Input: use a multi-part f-string shorter than line length

   sep= " "
   FSTRING = (
       f"many"
       f"{sep}"
       f"tiny"
       f"{sep}"
       f"fstrings"
   )

2. Run it locally or via this vercel example
3. The output wastes horizontal space: it puts them on one line, but not joined together

   sep= " "
   FSTRING = f"many" f"{sep}" f"tiny" f"{sep}" f"fstrings"

Desired style

sep = " "
FSTRING = f"many{sep}tiny{sep}fstrings"

Additional context
Originally found while reviewing a PR for Python Arcade, a game framework and educational tool.

[nottarun7]

Which file is the current function in or is there a need to create a new file?

[kastkeepitjumpinlikekangaroos]

hey! I've opened a PR here #4480 implementing a change for this issue FYI! Currently awaiting feedback (not sure what the standard protocol for issues like this which haven't been officially accepted yet but this seems to be a net improvement to the formatting IMO)

[cobaltt7]

This feature already exists in the --unstable style (string_processing feature)

See #4208

[kastkeepitjumpinlikekangaroos]

ah thanks @cobaltt7 - will close the PR then!