feat(bundle): support html entrypoint (#29856)

For instance

`deno bundle --outdir dist index.html`

It will find scripts referenced in the html, bundle them, and then
update the paths in index.html for the bundled assets.

Right now it doesn't handle other assets (from `link` elements), but it
could

tests/specs/bundle/html/imports-css.ts

@@ -0,0 +1,3 @@
+import "./style.css";
+
+document.body.insertAdjacentHTML("beforeend", "<h1>Hello, world!</h1>");