Improve pullquote block styles

[mikachan]

This PR improves the pullquote block styles:

• Applies small font size to citation
• Adds top and bottom border
• Applies 1.3 line-height
• Adds top and bottom margin using the var(--wp--preset--spacing--40) preset

Closes #263.

[beafialho]

Thank you @mikachan! I have only two comments:

• In the front end, everything looks ok but citation should not be Italic, it should be Regular

• In the editor, the citation is appearing all caps (it should not be)

[mikachan]

Thanks for taking a look! These two things should be fixed now. I've also improved the citation styles for the quote block, so they're the same as the pullquote.

I had a go at improving the default quote appearance too. I can't seem to improve the border, as the border should change with the quote alignment (left border for left alignment, right border for right alignment), and we can't control this in theme.json. However, adding padding makes the quote at least look different, while we wait for WordPress/gutenberg#44640 to land: