[Navigation] Align Navigation after rem update

[mrcthms]

WHY are these changes introduced?

The recent Nav density work updated spacing in the Navigation, but after the recent base font size update, things are looking a little wonky. This PR fixes the wonkiness in the Navigation and aligns it as it should be.

WHAT is this pull request doing?

Updates include:

• Correct margin between edge of NavigationItem and icon
• Correct margin between icon and text within NavigationItem
• Correct padding-left of a SecondaryNavigation to align with text within an NavigationItem
• Correct padding-left of a SectionHeading to align with the icon

🎩 checklist

• Tested on mobile
• Tested on multiple browsers
• Tested for accessibility
• Updated the component's README.md with documentation changes
• Tophatted documentation changes in the style guide
• For visual design changes, ping @ sarahill to update the Polaris UI kit

[github-actions]

size-limit report

Path	Size
cjs	164.56 KB (0%)
esm	94.3 KB (0%)
esnext	141.38 KB (-0.02% 🔽)
css	34.64 KB (-0.05% 🔽)