Nixpkgs switched pkgs.formats.yaml to remarshal v2. Generated files now
start with a %YAML 1.1 directive and a --- document start, and multiline
strings use double-quoted escapes.
Only files written through yamlFormat.generate change. Modules that
declare the YAML type but write the file with builtins.toJSON keep their
current output.
Add an optional session setting so that session names can be different
than the smug project name. This also enables using template variables
when naming sessions.
