jsx-to-htm: leading underscore = reference tag

In JSX, a leading underscore in tag names denotes a reference tag (Component):

```js
var x = <_Foo />
// should output:
var x = html`<${_Foo} />`
```

Author: developit

packages/babel-plugin-transform-jsx-to-htm/index.mjs

@@ -92,7 +92,7 @@ export default function jsxToHtmBabelPlugin({ types: t }, options = {}) {
 		const open = node.openingElement;
 		const { name } = open.name;
 
-		if (name.match(/^[A-Z]/)) {
+		if (name.match(/^[_A-Z]/)) {
 			raw('<');
 			expr(t.identifier(name));
 		}