Fix incorrect `stylelint-polaris` failures in `@font-face` rules

[aaronccasanova]

We are incorrectly failing font-weight usages in @font-face rules and recommending authors use Polaris custom properties. However, Polaris custom properties are not really applicable in this context and more importantly are not permitted in @font-face rules:

Possible solutions:

• Suggest authors add a stylelint-disable-next-line comment:
  • Easy to document
  • Concise documentation can be added to the stylelint error
  • @font-face usage is typically minimal
  • Low time investment
• Create a custom stylelint-polaris rule that handles the edge case:
  • No additional documentation required
  • Higher time investment
  • Rough outline:
    • Wrap the declaration-property-disallowed-list rule
    • Intercept font-weight failures
    • Check if the font-weight's parent is a @font-face node
    • Exit early

Note: Our styles-tokenize-font migration is also incorrectly replacing font-weight values with tokens.

[aaronccasanova]

Closed by #7993 and #7997