Add font smoothing + text link styles

[kjellr]

Fixes #108 and #161.

Loads the style.css file in the editor, and then adds CSS for font smoothing, and for basic text link styles (including hover, active, and focus). These styles do not impose a text-decoration: underline rule, so they won't effect or override elements that don't have an underline by default (like the button block).

Each group of rules is accompanied by a description of why it's implemented with CSS, and points to a Gutenberg issue for more details.

Screenshots

Link hover state:

Note: I went with a dashed instead of dotted underline so that it better matches the dashed lines used throughout the bird patterns.

Before	After

[kjellr]

Note that some of the underline rules can be added to theme.json via this technique, but it seems a little delicate to me, and we'd still require CSS for some other text link styles.

[jffng]

Some small changes, otherwise LGTM.