Add an `Iff` helper (#172)

I ran into some situation where I want to conditionally render a node if
some variable is not nil and obviously I got a panic

```go
// package viewmodels
type SomePage struct {
	s *string
}

// package views
func SomePage (vm viewmodels.SomePage) g,Node {
	return Div(
		If(vm.s == nil, Text("s is nil"),
		If(vm.s !- nil, Text("s is " + vm.s), // this will panic when `s` is nil
	)
}
```

In this situation, go will interpret the code of the second `if`
regardless of the condition because the code itself is not in a
condition.

This PR introduces a new `Iff` helper that accepts a callback. The
callback content is only interpreted when it's called, making the code
safe:

```go
// package viewmodels
type SomePage struct {
	s *string
}

// package views
func SomePage (vm viewmodels.SomePage) g,Node {
	return Div(
		Iff(vm.s == nil, func () g.Node { return Text("s is nil") },
		Iff(vm.s !- nil, func () g.Node { return Text("s is " + vm.s) },
	)
}
```

I'm aware of the `Lazy` effort on the side, but I guess this is no a
breaking change and can still exist in addition to the `Lazy` effort.

Co-authored-by: Markus Wüstenberg <markus@maragu.dk>

0 files changed, 0 insertions, 0 deletions