Type annotation for mathjs.unit is not correct

[mia-trilo]

Describe the bug
In index.d.ts, the unit function is annotated as unit(value: MathNumericType, unit?: string): Unit. However, according to the documentation, the valid calls are

math.unit(value: number, valuelessUnit: string) : Unit
math.unit(value: number, valuelessUnit: Unit) : Unit
math.unit(unit: string) : Unit
math.unit(unit: Unit) : Unit

I believe the unit function should be typed as unit(value: MathNumericType, unit?: string | Unit): Unit

[tusharAgarwal2511]

Hi, I would like to work on this issue if it's still open. I'm new to contributing, but I believe I can update the TypeScript definitions for unit() to match the documented overloads and submit a PR.