Provide a theme toggle plugin by default

[martinRenou]

Problem

As suggested in https://github.com/pydata/pydata-sphinx-theme/issues/745#issuecomment-1905560716

@jtpio just linked this in another thread https://jupyterlite.readthedocs.io/en/stable/howto/configure/advanced/iframe.html

It showcases an example making a jupyterlite plugin allowing to set the theme from outside of the iframe.

We could provide such a plugin by default in jupyterlite-sphinx, allowing pydata-sphinx-theme to notify jupytelite iframes of a potential theme change.

Not only We could support setting the theme using a theme name, but the message could probably also contain css variable values as a dictionary.