macro: no else support in conditionals

[dannywillems]

Description

`crates/ironhtml-macro/src/lib.rs` — `IfNode` generates `.when(cond, |e| e ...)` calls, which only handles the true branch. There is no `else` support even though the runtime API has `.when_else()`.

Users cannot write:
```rust
html! {
if #is_admin {
span { "Admin" }
} else {
span { "User" }
}
}
```