Rename path replace-extension to path with-extension, add with-stem and with-parent

[132ikl]

Extends #1002. Renames path replace-extension to path with-extension, in following with other languages (Rust, Python), and adds path with-stem and path with-parent. Also moves the path module into std-rfc so it can be used like use std-rfc/path.

Adds a private helper function, with-field, that with-extension, with-stem, and with-parent can use. These can each be dead simple functions, while giving users more options for path manipulation.

The motivation for separate with-extension, with-stem, and with-parent functions, rather than a more general function like path with is the following:

• with-extension has special behavior for stripping periods
• you can tab-complete path with<TAB> to immediately see all the possible options
• you can't accidentally pass an invalid field to path with
• there can be separate examples for with-extension, with-stem, with-parent for only the relevant functionality

[132ikl]

I can't request a reviewer for my own PR for some reason, so ping @WindSoilder 😅

[WindSoilder]

Oh, thank you for improving it!

[fdncred]

I don't think this output is what you'd get with the input above.

[132ikl]

whoops, forgot to copy the updated results back to the example. sorry for slandering your home directory 🤣 fixed in #1013