Twenty Nineteen: Typographic scale improvements

[kjellr]

As noted during the early development of Twenty Nineteen, there are a couple improvements to the font size and typographic scale:

• The theme sets a base font size of 22px. Ideally, this should be set to 100%. ( Discussion: ​https://github.com/WordPress/twentynineteen/issues/62 )
• The em-based typographic scale built on top of that 22px size does not always resolve to exact pixels. ( Discussion: ​https://github.com/WordPress/twentynineteen/issues/164 )

It's worth noting that adjusting this scale at this point must:

1. Preserve the current appearance. Font sizes must be close enough to what's live currently that they don't appear visually different. Adjusting our math so that type sizes are rounded up/down to the nearest pixel when rendered is fine, but we don't want to implement a whole new, different type scale.
2. Use pixel or rem-based sizes in the editor styles in order to override the default pixel-based font sizes used in Gutenberg.