Add hydrate directives

[hawkticehurst]

Feature request

Similar to Astro client directives or 11ty is-land implement some sort of hydrate directive(s) for hydrating Stellar components at different times/places –– not just page load.

Potential directives:

• hydrate:load – hydrate immediately on page load (default)
• hydrate:visible – hydrate once the component is visible in the viewport
• hydrate:media – hydrate based on a CSS media query
• hydrate:interaction – hydrate when the component is interacted with (i.e. click, mousenter, etc)

Open questions:

Should we implement this functionality at all? Could we instead create custom Astro client directives or just recommend that people use is-land? More exploration/research should be done to see what the options are.