simplify basic usage

[WickyNilliams]

i'm thinking we can simplify basic usage, so that people don't need to render their own <calendar-months> with offsets etc.

this would simplify the basic case (and probably be good enough for most people's needs), whilst preserving the ability to have more advanced layouts where needed

as a result, it would mean these are equivalent:

<calendar-date months="2"></calendar-date>

<calendar-date months="2">
  <calendar-month></calendar-month>
  <calendar-month offset="1"></calendar-month>
</calendar-date>

to be sensisitive to perf, we should only render into the default slot if nothing is slotted