rustdoc: dedup search form HTML

This change constructs the search form HTML using JavaScript, instead of plain HTML. It uses a custom element because

- the [parser]'s insert algorithm runs the connected callback synchronously, so we won't get layout jank
- it requires very little HTML, so it's a real win in size

[parser]: https://html.spec.whatwg.org/multipage/parsing.html#create-an-element-for-the-token

This shrinks the standard library by about 60MiB, by my test.

tests/rustdoc-gui/javascript-disabled.goml

@@ -4,7 +4,7 @@ javascript: false
 
 go-to: "file://" + |DOC_PATH| + "/test_docs/struct.Foo.html"
 show-text: true
-assert-css: (".sub", {"display": "none"})
+assert-false: ".sub"
 
 // Even though JS is disabled, we should still have themes applied. Links are never black-colored
 // if styles are applied so we check that they are not.