Add support for defining fontSizes

[kaltoft]

Description

This PR adds support for defining font sizes in the theme. With this implementation fontSize is strictly set by referencing theme tokens, like with colors and spacing.

const theme = createTheme({
  fontSizes: {
    title: 34,
    h1: 28,
    h2: 22,
    h3: 20,
    body: 16,
  },
  textVariants: {
    defaults: {
      fontSize: 'body',
    },
    heading: {
      fontSize: 'h1',
    },
});

Checklist

• I have added a changelog entry following the Keep a Changelog guidelines.

[kaltoft]

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