Add types from index to quarterOfYear

[bensaufley]

Resolves an issue where a variable being passed in as unit that may conform to either the base function's expected type (e.g., ManipulateType) or the augmented function's type (QUnitType) would be considered a type violation:

type MyUnit = 'days' | 'weeks' | 'months' | 'quarters' | 'years';

const doThing = (date: Dayjs, unit: MyUnit) => {
  if (date.isSame(dayjs(), unit)) {
    return date.add(7, unit);
  }
  return date.subtract(3, unit);
};

The types as they stand currently have two overloads: one that takes the base types and one that takes the extended types; neither take both, when really, the plugin makes it so that the function can take both kinds. This causes an error when I may pass in 'quarters' or 'weeks' because QUnitType adds 'quarters' to UnitType and OpUnitType/ManipulateType adds 'weeks' but there is no type that has both.

This PR could also add this as a separate overload, if that were important. E.g.:

add(value: number, unit: QUnitType): Dayjs
add(value: number, unit: QUnitType | ManipulateType): Dayjs

But I don't think it is – I don't see any value in an overload only accepting QUnitType.

Another open question, I suppose: here are the types today:

  export type OpUnitType = UnitType | "week" | "weeks" | 'w';
  export type QUnitType = UnitType | "quarter" | "quarters" | 'Q';
  export type ManipulateType = Exclude<OpUnitType, 'date' | 'dates'>;

Should the QUnitType being passed to add/subtract also be Manipulated, excluding 'date' | 'dates'? Are 'date' | 'dates' valid values for add/subtract only when the plugin is added?

Maybe:

  export type OpUnitType = UnitType | "week" | "weeks" | 'w';
  export type QUnitType = UnitType | "quarter" | "quarters" | 'Q';
  export type ManipulateType = Exclude<OpUnitType, 'date' | 'dates'>;
  export type ManipulateQType = Exclude<QUnitType, 'date' | 'dates'>;

And maybe QUnitType should extend OpUnitType rather than UnitType? Which would make most of the changes in this PR obsolete, except that we'd replace add#unit/subtract#unit with ManipulateQType?

[bensaufley]

OK I've pushed an update after kind of rubber-duckying myself in the PR notes. Take a look at either commit and lmk which solution you prefer. I think I like the latest one best.

[bensaufley]

It looks like Codecov is failing, not the tests? I don't think I've done anything there.