Refresh emptytheme

[kjellr]

This does a few things to bring emptytheme more in sync with latest Gutenberg developments:

• It changes the theme.json version to 2.
• It uses "appearanceTools": true instead of opting into a bunch of different features.
• It changes block-templates and block-template-parts folders to templates and parts.
• It removes some theme_supports that now are automatically added to block themes.

[scruffian]

Thanks