Function create_sprint 

pub async fn create_sprint(
    project_dir: &Path,
    id: &SprintId,
    title: &str,
    goal: Option<String>,
    period: Option<(DateTime<Utc>, DateTime<Utc>)>,
) -> Result<Sprint>

Create a sprint on the board in project_dir and return the saved Sprint.

The state is crate::sprint::SprintState::Planned. goal is persisted after the frontmatter as the sprint Markdown body. When period is provided, retain the planned start and end dates. Return Error::InvalidSprintPeriod when the start is after the end, Error::SprintExists when the ID is already used, Error::NotInitialized for an uninitialized board, or Error::EmptySprintTitle for an empty title.