feat: Add border tokens and use rems for widths

[alimpens]

• I moved the border tokens from common to brand, they make more sense there imo
• Negative values don't seem to work with these tokens, so Tabs button still has a negative hardcoded value
• In Table, the border is set on the cell, because a border on the row doesn't work with the default border-collapse: separate. border-collapse: collapse caused other issues I think, but I can't quite remember what they were.