Support `::first-letter` and `::first-line` pseudo styles

[nicoburns]

These pseudo-elements allow different styles to be applied to the first letter or first line of a layout.

• The first letter is the first grapheme along with surrounding punctutation.
• The first line is the first line according to line-breaking

See: https://drafts.csswg.org/css-pseudo/#typographic-pseudos

[DJMcNab]

It seems to me that ::first-letter is trivial to implement outside of Parley itself. But I do agree that we clearly need an answer for how someone does ::first-line.