fix(types): mark MagicString options as optional

[lukeed]

Without this, something like new MagicString('...', { filename }) is marked as invalid

[rileyajones]

👍

[aleclarson]

This is ready to merge 👍

[ecstrema]

Indeed. Ready to merge. Would be nice to have.

[antfu]

Released as v0.25.8

[privatenumber]

FWIW, I think the Partial<> should be directly applied to MagicStringOptions (or better yet, manually add ? to each property) for a consistent source of truth.

Otherwise, the exported type needs to be Partial'd every time it gets used (eg. when creating a MagicString wrapper).