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

[kastkeepitjumpinlikekangaroos]

Description

Hey! First time contributing to the project so please let me know if I'm doing anything wrong here :)

I've implemented changes for the issue described here #4389

Esentially if we can combine multiple f-strings into a single f-string without breaking other considerations like the intended line length then I attempt to do so. This is done by breaking apart the AST of multiple f-strings into a single f-string node by only keeping the first FSTRING_START token and the last FSTRING_END token and keeping all tokens inbetween.

Checklist - did you ...

I've added 2 new test cases showing that the examples in the issue are fixed by the change. All other tests are passing which indicates that the change does not affect other cases.

• Add an entry in CHANGES.md if necessary?
• Add / update tests if necessary?
• Add new / update outdated documentation? (let me know if there's some place I should add this!)