Add typescript child type checking

Closes #1086

It fixes the original issue from #1008, it will now type check correctly

It also allows type inference (and checking) of function bodies, a basic
example is given in the docs that infers the type of `num` but anything
like preactjs/preact-router#269 can benefit from this

src/preact.d.ts

@@ -4,8 +4,8 @@ export as namespace preact;
 declare namespace preact {
 	type Key = string | number;
 	type Ref<T> = (instance: T) => void;
-	type ComponentChild = JSX.Element | string | number | null;
-	type ComponentChildren = ComponentChild[];
+	type ComponentChild = VNode<any> | string | number | null;
+	type ComponentChildren = ComponentChild[] | ComponentChild | {} | string | number | null;
 
 	/**
 	 * @deprecated
@@ -104,12 +104,12 @@ declare namespace preact {
 	function h<P>(
 		node: ComponentFactory<P>,
 		params: Attributes & P | null,
-		...children: (ComponentChild | ComponentChildren)[]
+		...children: ComponentChildren[]
 	): VNode<any>;
 	function h(
 		node: string,
 		params: JSX.HTMLAttributes & JSX.SVGAttributes & Record<string, any> | null,
-		...children: (ComponentChild | ComponentChildren)[]
+		...children: ComponentChildren[]
 	): VNode<any>;
 
 	function render(node: ComponentChild, parent: Element | Document, mergeWith?: Element): Element;
@@ -136,6 +136,10 @@ declare global {
 			props: any;
 		}
 
+		interface ElementChildrenAttribute {
+			children: any;
+		}
+
 		interface SVGAttributes extends HTMLAttributes {
 			accentHeight?: number | string;
 			accumulate?: "none" | "sum";

test/ts/VNode-test.tsx

@@ -57,3 +57,9 @@ describe("VNode", () => {
 		expect(comp.children[1]).to.be.a("string");
 	});
 });
+
+class TypedChildren extends Component<{children: (num: number) => string}> {
+	render() { return null }
+}
+
+const typedChild = <TypedChildren>{num => num.toFixed(2)}</TypedChildren>