feat: switch font-loading to prefetch. Add font weight option

[vpicone]

Closes #48

In this PR we switch to pre-fetching the google fonts to improve font-loading performance and also to offer weights other than those provided by the @carbon/type package.

To that end, this PR also also allows devs to configure additionalFontWeights other than the defaults as a theme option:

// gatsby-config.js
__experimentalThemes: [
    {
      resolve: 'gatsby-theme-carbon',
      options: {
		// will get added to default [300, 400, 600]
        additionalFontWeights: [200]
      },
    },
  ],

[vercel]

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[alisonjoseph]

nice! 😍