fix: TypeScript declarations

[simon-lammes]

Previously, the functions fileOpen and fileSave used an undefined generic type of O. This caused TypeScript compiler issues in some environments. (details in commit discussion)

If I understand the types correctly, the generic type O is not needed. We just want TypeScript to know that options is either of the array type [FirstFileSaveOptions, ...CoreFileOptions[]] or of the object type FirstFileSaveOptions.

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[simon-lammes]

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[tomayac]

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