add digital climate strike banner

[Rich-Harris]

This adds a fullscreen banner that will activate tomorrow (20 Sep) to encourage people to take part in the Digital Climate Strike. The idea is to show solidarity with people striking IRL, and to encourage people — if they can — to do something other than be on a computer, since computers are a major factor in our impending doom (fun fact: data centres have a worse carbon footprint than the airline industry).

Of course, people can hide the banner iframe with devtools if they really need to access https://svelte.dev. But I think this is a movement worth supporting, however symbolically, and one that's aligned with Svelte's 'spend less time writing code and let browsers spend less time running code' goals.

[Swatinem]

fun fact: using svelte instead of other frameworks improves on this problem, since the code has less footprint both in the cloud (ssr) as well as client devices. :-)