Add preprocessor sourcemap

[benmccann]

No description provided.

[Conduitry]

This should be implemented in such a way that it does not fail with older versions of Svelte, and it doesn't look like this was done. The compiler throws an exception on unknown compiler options. If you have to bump the version of Svelte in order for existing tests to pass, then as implemented this is a breaking change.

[Conduitry]

Did the response from svelte.preprocess change in 3.30.0, or was the only change in svelte.compile? It may not have changed. The only way to do this might, unfortunately, be to check the VERSION of the copy of the compiler we're using. The other option is to update the peerDependencies and make this a major bump.

[benmccann]

I believe the response to svelte.preprocess did change in 3.30:

https://github.com/sveltejs/svelte/pull/5584/files#diff-0807847b16925d369083bb663f10c695f477946c95824dc38aeb9a17969e52d7R223

[benmccann]

I think this should be okay because it would pass sourcemap: undefined on < 3.30. I didn't manually test that though. Do you still think this is something that I should take a closer look at?

[Conduitry]

The compiler throws if an unknown key is present in the options object, even if its value is undefined.

[benmccann]

updated. thanks for taking a look!