Conditional slot errors at runtime

[mrkishi]

Slot content added inside an if currently fails at runtime:
https://svelte.technology/repl?version=1.39.2&gist=d4a50532f6a783ada53f35a2b9a7f9e4

This can be easily fixed by moving the condition inside the slot:
https://svelte.technology/repl?version=1.39.2&gist=3bf75a560243feecdeaca013a0e7ad26

I don't know if it makes sense to allow slots inside conditionals, but if not, Svelte could at least error on their usage.

[Rich-Harris]

Thanks. I concluded that it's a bit too fiddly to allow slot='...' inside if/each blocks, so as of 1.39.3 this will cause a compile time error:

[mrkishi]

You really are unstoppable. 🥇