for_each: Substitution names

[Lestropie]

Despite having re-written the command for Python, I still find myself having to cross-check / test what substitution strings correspond to what components of input paths. I was reading a random pathlib Twitter/X post that came into my feed given I have to do #2931, and the thought came to mind to check for accepted conventions on names for components of filesystem paths, as maybe there are more robust definitions for use.

• "PRE": Both std::filesystem and pathlib refer to a path sans directory and sans file extension as the "stem". "STEM" would I think be better.

• Could add "PARENT" to refer to parent directory, ie. path sans file basename.

• "NAME" (possibly avoiding committing to either "basename" or "filename") could potentially do with cross-checking regarding how it behaves if the input is a directory path, with corresponding documentation.

[Lestropie]

Note that any modifications would need to be propagated to the ddocumentation.