opt into strict: true in tsconfig.json by default

[Rich-Harris]

closes #4395. This is a breaking change

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[dummdidumm]

This is closely related to #4495, we should at least release them together to give non-TS users a way to opt out.

The alternative would be to not add strict to the generated tsconfig, and remove checkJs as well. This means we would need to adjust #4495 to add strict accordingly.

[Rich-Harris]

Thinking further on this, it feels like strict probably belongs in the user-editable tsconfig/jsconfig, rather than the one we extend from. There's a few other compiler options I'd put in the same category

[dummdidumm]

Agree. I'd move the following to the user config:
skipLibCheck, esModuleInterop (honestly not even sure if it works for Svelte files since TS can't see the other files during transpiration)

The following I'm not sure about:
resolveJsonModule (if user doesn't have a declare module "*.json" somewhere they would get errors)

[Rich-Harris]

I don't think Kit or the default apps rely on resolveJsonModule though? Feels like that belongs to user config too

[Rich-Harris]

Closing this in favour of #4633, which documents the generated tsconfig and moves non-essential stuff into user config, where it can be changed freely