PowerShell’s ethos is that a module is the most granular unit of versioning and distribution. If you’re wanting to do anything more granular, then you’re more or less on your own, and working outside the shell’s native stuff. The reason it’s at the module level is because that’s what gets distributed to people; internally, for more granularity, Microsoft itself relies on their version control system if they need to retrieve previous versions, and refers to them by those builds. But an individual function isn’t seen as a standalone unit of work.
The most I can think of is a comment-based help field that you repurpose, or a private hashtable within each .ps1 that you put your own metadata into.
Personally, I’ve not taken this approach of a function-per-.ps1; a module is usually one .ps1 for me, and I test the whole thing. My tests are tagged, so if I know I only need to test a certain function, then I just have Pester test only the tests with that function’s tag.