Add new themeStore core module for managing theme state

[sebnitu]

What changed?

This PR adds the new themeStore core module for managing and maintaining theme state. It provides an API for setting a theme, adding and removing possible themes as well as callbacks for when the store is first initialized and when a theme changes. Theme state is persisted via local storage and theme classes are applied to the html element.

Additional changes

• cssVar now takes an options param and allows setting a fallback value for when a CSS custom property is not found.
• Adds prefix-themes to core css custom properties.
• Applies new markup and logic to the vrembem package's vite demo including a theme toggle and VITE_PACKAGE variable.