feat(button): add support for isSelected in low emphasis

[gcornut]

• Reworked button styles so that the selected state can apply on top of both medium and low emphasis
• Deprecate selected state padding variables as we prefer to keep the base emphasis style for sizing

Selected state in low and medium emphasis is testable here: https://4436b6bb--5fbfb1d508c0520021560f10.chromatic.com/?path=/story/lumx-components-button-button--state-variations

StoryBook: https://1e0d3d2e--5fbfb1d508c0520021560f10.chromatic.com/ (Chromatic build) ⚠️ Outdated commit

[gcornut]

Test OK
(tested in product directly)
Button medium selected work like before ✅
Button low selected work like before ✅ (but future proofed for the change on the border of the selected state)
(tested on product components: v1 follow button, wrex toolbar elements, v2 follow button)
Date picker selected date ✅ (basically a button low selected)