Use relative units for layout properties

[janogarcia]

As discussed internally, use relative units so that components can scale based on the base font size.

At component level

• Make sure we use rem for font size values and relative units for other typesetting properties (line height, letter spacing, etc.)
• Use rem instead of px for layout (spacing, min height, etc.). Exceptions to this rule may apply (setting a border to 1px that we don't want to scale with font size).

At the application level

These tasks are out of the scope of Compound components, but we can provide guidelines to be followed at the application level.

• Set HTML base font size to 100% instead of 16px. Refer to specific notes on implementation requirements.
• Use em for breakpoints.