Move library files into assets folder

Problem/Motivation

The CSS file has a body style that messes up site styling and requires an override. There's also some linting issues with the CSS file, although those don't affect any functionality.

Composer puts the library in the wrong place (/libraries/easy-responsive-tabs-to-accordion), so we have to override that with a libraries-override in the theme.info.yml file to get things to even load.

Proposed resolution

The library has a MIT license and is 9+ years old, and likely will never be updated, so we might as well just put both the JS and CSS files in the asset folder and save us the headache of pulling in the library at all.