feat(color-contrast): add px unit to error messages

[straker]

Add the size in px to the color contrast error message to avoid confusion with the pt unit.

"Fix any of the following:
  Element has insufficient color contrast of 3.87 (foreground color: #3d7e9a, background color: #eeeeee, font size: 12.0pt / 16px, font weight: normal). Expected contrast ratio of 4.5:1"

Linked issue: #1628

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• Follows the commit message policy, appropriate for next version
• Has documentation updated, a DU ticket, or requires no documentation change
• Includes new tests, or was unnecessary
• Code is reviewed for security by: @dylanb

[dylanb]

The commit should be marked as feat(color-contrast): because this is a new feature

[jeeyyy]

LGTM!

In my opinion this could read better as ${((fontSize * 72) / 96).toFixed(1)}pt (${fontSize}px)

Eg: 13.5pt (18px)

But that may be a personal preference.

[straker]

@JKODU I did that first, but since we were already in an parenthesis group I didn't know if that would be OK

(foreground color: #3d7e9a, background color: #eeeeee, font size: 12.0pt (16px), font weight: normal)

Updated PR title

[jeeyyy]

@straker, it is logical to express font as say 300 italic 1.3em/1.7em serif, which is why I thought parenthesis may be better. But this is a nitpick. Happy with what you decide.