Make selected a single value (not a length-1 array) if maxSelect=1

[janosh]

Closes #86.

@davipon I'd be curious to hear your opinion on this approach (since you expressed interest in #86).

The approach being to define a new internal _selected variable and making the exported prop selected reactive to it conditional on the value of maxSelect:

let _selected = selected as Option[]
$: selected = maxSelect === 1 ? _selected[0] ?? null : _selected

And the same for selected{Labels,Values}

  $: _selectedLabels = _selected?.map(get_label) ?? []
  $: selectedLabels = maxSelect === 1 ? _selectedLabels[0] ?? null : _selectedLabels
  $: _selectedValues = _selected?.map(get_value) ?? []
  $: selectedValues = maxSelect === 1 ? _selectedValues[0] ?? null : _selectedValues

[wd-David]

@janosh That's a brilliant way! Can't wait to reduce my boilerplates for single select.

[janosh]

Gotta love Svelte! ❤️ I expected the fix for #86 to take more work and break several tests. 😄

[janosh]

Released as v7.0.0. 🎉