Split html, css and json to basic and language feature extensions

[octref]

• html
• json
• css

[aeschli]

The new extensions are named html-language-features and json-language-features, matching markdown-language-features and typescript-language-features.
I also named the folders in the repo the same way (extensions/html-language-features). That's not consistent with markdown and typescript but I prefer keeping extension name and folder the same. @mjbvz What do you think, can we do the same for markdown and typescript?

[mjbvz]

I'll rename them too. I was originally concerned that it would mess up localization