Stabilize path_file_prefix feature

[Kivooeo]

This stabilises Path::file_prefix, following the FCP in tracking issue

(FCP ended almost a year ago, so if it's needed for proccess we could rerun it)

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[rustbot]

Some changes occurred in src/tools/clippy

cc @rust-lang/clippy

[jieyouxu]

FWIW there seems to be an outstanding concern #86319 (comment)

[Kivooeo]

I saw that but it seems like it's about adding another function, right? Or they want to make this new function a part of this stabilizaton?

[jieyouxu]

cc @the8472 (I don't know)

[the8472]

I was suggesting to make this part of this function. Alas, nobody responded to that suggestion.
It's rather late in the process and current API was already approved so you're not required to make that change, but I'll nominate it for discussion so the team can take another look at it.

[Kivooeo]

We can open FCP to add this as an additional function to this and make it part of this feature in this PR, because I also think that this is better to return both parts

This wasn't fully discussed at the time, so I think it's worth addressing now, and I believe it's better late than never