feat(components): align slotClassNames property for all components

[bmdalex]

feat(components): align slotClassNames property for all components

Description

This PR:

• introduces ability to select submenus from MenuItem components: MenuItem.slotClassNames.menu
• aligns slotClassNames property on composed components in order to be able to target subcomponents directly;
  e.g.: a Menu item can be targeted referencing:
  • it's className directly: MenuItem.className // even before this PR
  • the item as a slot of the parent component: Menu.slotClassNames.item // introduced in this PR

[codecov]

Codecov Report

Merging #1093 into master will increase coverage by 0.01%.
The diff coverage is 94.73%.

@@            Coverage Diff             @@
##           master    #1093      +/-   ##
==========================================
+ Coverage    82.1%   82.11%   +0.01%     
==========================================
  Files         717      717              
  Lines        8571     8584      +13     
  Branches     1233     1169      -64     
==========================================
+ Hits         7037     7049      +12     
  Misses       1518     1518              
- Partials       16       17       +1

Impacted Files	Coverage Δ
...ackages/react/src/components/Dropdown/Dropdown.tsx	54.14% <ø> (ø)	⬆️
packages/react/src/components/Menu/MenuItem.tsx	61.26% <ø> (ø)	⬆️
packages/react/src/components/Header/Header.tsx	100% <100%> (ø)	⬆️
packages/react/src/components/Tree/TreeItem.tsx	59.37% <100%> (+1.31%)	⬆️
packages/react/src/components/Menu/Menu.tsx	90.74% <100%> (+0.17%)	⬆️
...kages/react/src/components/Accordion/Accordion.tsx	65.3% <100%> (+0.72%)	⬆️
packages/react/src/components/List/List.tsx	71.23% <100%> (+0.39%)	⬆️
...ges/react/src/components/RadioGroup/RadioGroup.tsx	95.16% <100%> (+0.07%)	⬆️
packages/react/src/components/Tree/Tree.tsx	75% <100%> (+0.58%)	⬆️
packages/react/src/components/Form/Form.tsx	100% <100%> (ø)	⬆️
... and 2 more

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