feat: add with_replaced_extension as a alias of with_extension

[anatawa12]

This PR adds Path::with_replaced_extension that is exactly same as Path::with_extension but with proper name.

This PR implements some portion of my suggestion about path_add_extension feature since one positive reaction are received.
#127292 (comment)

Currently, this new function is under path_add_extension feature but it might be better to move under another feature.
Should I split feature flag?

After thinking a while since I suggested, I think PathBuf::set_extension is not as ambiguous as Path::with_extension so I don't added here.
However not adding replace_extension would be a little inconsistent so there is possibility to be better to add PathBuf::replace_extension.
I want to ask for community and library team.

Relates to path_add_extension feature which is tracked in #127292

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r? @Mark-Simulacrum

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