Update Backbone.sync to use a union
Update Backbone.sync to use a union

Updating file to use union to better follow the typescript Do's and Don'ts under the 'Use Union Types' section found in the link below.
https://www.typescriptlang.org/docs/handbook/declaration-files/do-s-and-don-ts.html

This will help prevent issues where a line of code such as 'let proxiedSync = Backbone.sync;' will use the wrong sync function.
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