feature: diff & add functions

[WilcoSp]

This PR is stacked on PR https://github.com/formkit/tempo/pull/58

The diff & add function I had mentioned before at https://github.com/formkit/tempo/pull/66#issuecomment-2263626679

This PR adds the following:

• diff function which takes 1/2 date(s) + optional options to return an object which can be used with Intl.DurationFormat.prototype.format()
• add function which takes optionally a date, a DurationObj & optional a boolean to overflow the date to the next month, to return a new Date object

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