Components and children

@Name(args) renders another component; a trailing block passes children, rendered where the callee says { children... }. The legacy {! expr } call form from the baseline is retained.

templ layout() {
	<main>{ children... }</main>
}

templ page() {
	@layout() {
		<p>body</p>
	}
}

AST: TemplElementExpression, ChildrenExpression, CallTemplateExpression (legacy form).

Verified by: internal/generator/test-call, internal/generator/test-templ-element, internal/generator/test-go-template-in-templ, internal/generator/test-templ-in-go-template, internal/generator/test-context, internal/generator/test-cancelled-context.