[fix] ReadOnlyFormData types

[JeanJPNM]

Fixes types of the ReadOnlyFormData interface.
The changes allow instances of the class and its methods to be used inside a for ... of loop without receiving an error like:
TS2495: Type 'ReadOnlyFormData' is not an array type or a string type.

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Latest commit: 8b061b4

The changes in this PR will be included in the next version bump.

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@sveltejs/kit	Patch

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[dreitzner]

lgtm

[dummdidumm]

Please add a changeset, then we'll merge