Invalid "sortField" property type in Vue and type definition files

[nestorrente]

This issue is related to #647. In that issue, @richardmward pointed that sortField prop of DataTable component was declared as String in the .vue file - that was fixed in version 3.0.2. However, the same issue is present in TableHeader and Column components. Also, Column.d.ts file has sortField defined as string, instead of string | Function.

The same issue is also present in DataTable, TableHeader and Column components of version 2.x - also in their corresponding type declaration files.

Refering to TypeScript definition files, it might be better to define it as string | ((item: any) => any), in order to give a little more information about function's signature.

Thank you very much for your time.

[cagataycivici]

Thank you!