feature request: add .Clear method

[novadev94]

Currently, it's impossible to clear the OrderedMap keeping its capacity.

A simple implementation would be:

func (l *list[K, V]) Clear() {
	l.root = Element[K, V]{} // just in case K & V are pointer types
}

func (m *OrderedMap[K, V]) Clear() {    
	clear(m.kv)
	m.ll.Clear()
}

[elliotchance]

Happy to accept a PR for this. I see clear() was introduced in Go 1.21, so for backwards compatibility it shouldn't use that function in this implementation.