Function add 

pub fn add(
    manifest_path: &Path,
    source: &str,
    name: Option<&str>,
    variants: &[String],
    size: Option<u16>,
    force: bool,
) -> Result<Vec<String>, AddError>

Writes a new entry into icons.gui.toml, preserving everything else in the file (toml_edit, not guicons-core’s read-only toml_span parser). Returns the manifest keys that were added.