Template declarations
templ Name(params) { … } declares a component returning
ghtmx.Component. Methods (templ (r Receiver) Name() { … }) and
type parameters are supported.
templ hello(name string) {
<div>Hello, { name }</div>
}
AST: HTMLTemplate; its body is a Nodes list of Nodes
(CompositeNode for the block-shaped ones).
Verified by: internal/generator/test-html,
internal/generator/test-method.