New formatter: gersemi

**Is your feature request related to a problem? Please describe.** This issue is a request for a formatter to be added: [gersemi](https://github.com/BlankSpruce/gersemi) (for CMake-related files) **Describe the solution you'd like** I would like for gersemi to be added as a formatter to treefmt-nix **Describe alternatives you've considered** - N/A (I already made a PR) **Additional context** - [github:BlankSpruce/gersemi](https://github.com/BlankSpruce/gersemi) - [NixOS Package Search - gersemi](https://search.nixos.org/packages?channel=25.11&query=gersemi)