chore: default options.filename to "(unknown)"

[Rich-Harris]

just makes like a little simpler and more consistent (without a breaking change) — closes #12980

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[dominikg]

does (unknown) end up in output like sourcemap or the svelte_meta on nodes during dev? that could lead to subsequent errors, although i am not sure how well these would fare with the current undefined.

[Rich-Harris]

Probably. But if you're not supplying a filename it's because you're doing things by hand (no bundler plugin etc) and need to understand what you're doing anyway. The alternative would be to make the filename option required

[trueadm]

I think we should probably try and make it required but this is a good half-way step.