Remove the theme information from style.css and add a theme manifest file

[jolyonruss]

Currently the metadata related to a theme is store in a comment at the top of style.css.

I argue this is not a clean separation of concerns, yes .css files can contain semantic information to help the reader navigate the file, but using it to store metadata is bad practice.

I propose creating a theme manifest file that sits at the root of the theme directory and contains all the metadata related to the theme that's currently at the top of style.css, things like the name, author, etc. but it could also contain a file list for the theme with a few lines about the files usage.

Externalising the metadata and adding more contextual information will greatly improve developers ability to hack on top of others themes and understand their rationale for certain decisions.

[ocean90]

Duplicate of #25927.