Rollup merge of #145209 - hanna-kruppe:path_add_extension, r=tgross35

Stabilize `path_add_extension`

FCP in tracking issue rust-lang/rust#127292 finished in January. There was more discussion since then, but [libs-api decided to match `set_extension`][0] by keeping the return type as-is and adding a panic for invalid extensions. The latter was implemented in rust-lang/rust#140163, so this feature should be ready for stabilization. But if anyone's unsure, another FCP could be done to confirm.

Closes rust-lang/rust#127292

[0]: rust-lang/rust#127292 (comment)

Author: Zalathar