fix(Dropdown): fix switch between multiple and single modes

[layershifter]

Fixes #1658.

Fix 🔧

Previously, we had value in the Dropdown's state that conditionally can be single item or array of items. This PR performs normalization of values, so we will always deal with array of items. This allows to remove a lot of type casts and conditions.

A code snippet to test

import React from 'react'
import { Checkbox, Dropdown } from '@stardust-ui/react'

const inputItems = [
  'Bruce Wayne',
  'Natasha Romanoff',
  'Steven Strange',
  'Alfred Pennyworth',
  `Scarlett O'Hara`,
  'Imperator Furiosa',
  'Bruce Banner',
  'Peter Parker',
  'Selina Kyle',
]

const DropdownExample = () => {
  const [search, setSearch] = React.useState(false)
  const [multiple, setMultiple] = React.useState(false)

  return (
    <>
      <Dropdown items={inputItems} multiple={multiple} search={search} />
      <Checkbox
        checked={multiple}
        label="multiple"
        onChange={(e, props) => setMultiple(props.checked)}
      />
      <Checkbox checked={search} label="search" onChange={(e, props) => setSearch(props.checked)} />
    </>
  )
}

export default DropdownExample

[codecov]

Codecov Report

Merging #1830 into master will increase coverage by 0.01%.
The diff coverage is 97.43%.

@@            Coverage Diff             @@
##           master    #1830      +/-   ##
==========================================
+ Coverage   69.52%   69.53%   +0.01%     
==========================================
  Files         875      875              
  Lines        7609     7590      -19     
  Branches     2225     2210      -15     
==========================================
- Hits         5290     5278      -12     
+ Misses       2311     2304       -7     
  Partials        8        8

Impacted Files	Coverage Δ
...ackages/react/src/components/Dropdown/Dropdown.tsx	88.04% <97.43%> (-0.18%)	⬇️
...rc/themes/teams/components/Alert/alertVariables.ts	0% <0%> (ø)	⬆️
...test/specs/commonTests/implementsShorthandProp.tsx	100% <0%> (ø)	⬆️
...t/src/themes/teams/components/Alert/alertStyles.ts	7.69% <0%> (+0.54%)	⬆️
packages/react/src/lib/renderComponent.tsx	86.44% <0%> (+1.19%)	⬆️
packages/react/src/components/Alert/Alert.tsx	100% <0%> (+20%)	⬆️

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