dayjs 2.0 question - functionality of QuarterOfYear plugin vs WeekOfYear plugin

[BePo65]

I'm currently implementing the QuarterOfYear plugin for dayjs 2.0 (as a sample for a plugin that overloads existing Dayjs methods). While on it, I noticed that the question of what functionality of a plugin for "new a unit" is implemented in what module is not consistent between different plugins.

To make clear what I want to say, here a little table:

	QuarterOfYear	WeekOfYear	IsoWeek
get / set	in plugin (quarter)	in plugin (week, weeks)	in plugin (isoWeek)
add / subtract	in plugin	in dayjs core	not implemented
diff	in dayjs core	in dayjs core	not implemented
startOf / endOf	in plugin	in dayjs core	in plugin (new unit 'isoweek')
remarks		plural (weeks) part of plugin	no plural in plugin pluralGetSet; no short unit

IMO for dayjs 2.0 we should be more consistent.

The question is: what is the "correct" implementation?
For QuarterOfYear I implemented both ways, but I only created a pr #2053 for the way 'WeekOfYear' does it (just implementing get / set).

So what should we do - stay with the simple get / set implementation of 'WeekOfYear' or change it to the 'full implementation' with all methods implemented in the plugin, overloading the methods in the core module.