More Visual Refresh Tweaks

[emplums]

This PR makes the tweaks listed out in #731 ✨

• Update selected style for primary and outline box shadow to 2px instead of 1px
• Update UnderlineNav & SideNav states (removed bar on the left on hover/focus)
• SideNav should have had a white background, but it was grey so I updated that
• The grey background hover on SideNav items was overlapping the border radius of the container, so I added border radius to the first and last SideNav items
• Fix SelectMenu tab styles
• disabled state for buttons with color icon
• Change back the text in labels and topic tags from font-weight 500 to 400
• Add label colors to the theme
• Spacing between select menu header and select menu tabs was mistakingly set as 24px. I have no changed it to 16px.
• Update blue text in Links, Outline button to blue[5]
• A bunch of button tweaks: https://github.com/github/design-systems/issues/770#issuecomment-605783773

Notes

• I added the label colors to the theme so that folks can pull from the theme if they'd like to use them, but we might want to think about whether or not we want to add some kind of colorScheme prop in the future to make using these easier.

• I removed primer-colors and primer-typography and moved over to @primer/primitives

• I had one question for @auareyou about the active/selected/hover state for SideNav that I left in the comments

Merge checklist

• Added or updated TypeScript definitions (index.d.ts) if necessary
• Added/updated tests
• Added/updated documentation
• Tested in Chrome
• Tested in Firefox
• Tested in Safari
• Tested in Edge

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[BinaryMuse]

Nothing to add, looks great! Thanks for replacing the old primitives.

[emplums]

Going to release this without the Avatar changes as @colinkeany mentioned it will be a bit more complicated than just adjusting the border radius across the board. We can do this in a different PR/release.

[emplums]

Going to do the Flash updates in a separate branch as well, as we should probably do an API refactor of Flash to use standard icons for success/failure/warn