feat: warn on obvious legacy component instantiation

[dummdidumm]

Adds a compiler warning that warns about legacy component instantiation (i.e. using new Component(..)). This won't catch all cases, but the most obvious ones which probably make up ~80%

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[Rich-Harris]

False negatives are okay but we should be careful to avoid false positives.

Are we concerned that people might rely on extensionless imports, i.e. importing file.svelte.js as ./file.svelte?

[paoloricciuti]

Are we concerned that people might rely on extensionless imports, i.e. importing file.svelte.js as ./file.svelte?

In my experience it really depends on the editor and tsconfig setup...I let my editor auto complete and sometimes it import extension less. We could change the generated tsconfig in sveltekit so that import the .js but I don't know if there's also some editor settings.

[Rich-Harris]

Yeah, I mean it even works in our own playground. I'm not sure we can realistically make this change. This feels like something that should be solved with typechecking, rather than in the compiler itself.

[dummdidumm]

I forgot the "is a default import" check, which silences all the false positives except the import DefaultClass from './module-which-looks-like-a-svelte-component.svelte, which I think will be super rare.
We can make it even more rare by checking that the new expression is called with exactly one argument.
Relying on types is not an option because we have to make it backwards compatible on a type level due to the componentApi option.

[Rich-Harris]

Fair enough. Should we go one step further and check that if the sole argument is an object literal, it has a target property since that's a required option if it's a component?

[dummdidumm]

yeah sounds good - it's very rare that you would pass a variable into the constructor that takes care of that for you