feat: Change if / else blocks logic

[cernec1999]

Currently, we construct if / else statements separately:

/// Creates a new if statement
pub fn new_if<C, E>(condition: C, then_block: Vec<E>) -> ControlFlowNode
where
    C: Into<ExprKind>,
    E: Into<AstKind>,
{
    ControlFlowNode::new(
        ControlFlowType::If,
        Some(condition),
        then_block
            .into_iter()
            .map(Into::into)
            .collect::<Vec<AstKind>>(),
    )
}

/// Creates a new else statement
pub fn new_else<T>(else_block: Vec<T>) -> ControlFlowNode
where
    T: Into<AstKind>,
{
    ControlFlowNode::new(
        ControlFlowType::Else,
        None::<ExprKind>,
        else_block
            .into_iter()
            .map(Into::into)
            .collect::<Vec<AstKind>>(),
    )
}

Instead, we should shift to a unified if / else block with an optional else body if one exists. This will make transformations easier.

We will need to change ControlFlowNode to reflect this change.