Nav sidebar: align visual styling closer to core and woo

[p-jackson]

Omnibus issue to address some of the current style issues in the nav sidebar and move it closer to what the rest of the core block editor looks like.

Mockup:

Specifically we should update

• padding around text and headings
• font size and colour
• styling of the selected item (blue instead of black)
• cut the animation time in half (currently using 200ms, the core <Modal> component uses 100ms)
• update the back CTA (i.e. the one to go back to calypso)

Part of #41298

[p-jackson]

Screenshot doesn't show an example of the hover style, but I think we can take some inspiration from the inserter menu, which changes the item's text to blue and adds a blue border on hover.

[p-jackson]

Latest mockup:

More here: pbAok1-185-p2#comment-2345