Move property mock logic to Twig compiler

[squrious]

Resolves #41

New release 2.21 of UX Twig Components removes the ability to override this and computed keys in the component template context.

The mock system was built on top of this ability, so as stated in the linked issue it can not work anymore this way.

This PR moves the mock logic to the compile step instead. When compiling templates for Storybook, Twig will add a special node to all module nodes to change the context before starting to display the actual node content. It's quite rough but as we are in a dedicated Twig environment, I consider it acceptable.

Then, when Storybook renders a component, it adds some context variables to the component event (a Storybook context) so the context changes are triggered from the compiled template based on these variables.

If it works well with the tag component syntax (embedded), the function syntax still triggers the base component renderer, which actually uses the base Twig environment, not configured for Storybook. There is a quick fix for this here to provide a different implementation of the component runtime. I hope we'll find a more robust way to do this in the future.