chore: enable hydration for declarative shadow roots

[vladitasev]

Declarative shadow DOM

Currently UI5Element.ts always calls attachShadow in the constructor, which in turn always creates a new shadow root (and destroys the existing declarative shadow root, if provided).

This change introduces a small fix to not destroy declarative shadow roots.

Notes about using declarative shadow DOM

1. Do not forget to pass all shadowRootOptions to the <template>, f.e. for Button.ts we have:

@customElement({
	...
	shadowRootOptions: { delegatesFocus: true },
})

therefore it needs:

<template shadowrootmode="open" shadowrootdelegatesfocus="true">...</template>

as opposed to the most commonly used:

<template shadowrootmode="open">...</template>

2. The initial rendering will always be executed, and will ideally only attach event listeners (the pre-rendered HTML is expected to be correct)

3. It's strongly recommended to use:

<style>
	*:not(:defined) {
		display: none;
	}
</style>

when rendering with declarative shadow DOM.