Improve the language syntax/format for designing UI

## What problem does this solve or what need does it fill?

The first time I started bevy ui, I found it's logic quite like html and css. But currently the code for building up ui are quite verbose and a hard to read once the ui panel get complicated.

## What solution would you like?

Use html and css for ui designing. And parse them to entities and components.

## Additional context

The `bevy_ecss` crate is using css to change the style of ui and it's much elegant and flexible than rust code. But just as its name, it does not support html to define the structure.

Edit:
Not just html, seemy comment below.